Interactive identification key to all brittle star families (Echinodermata; Ophiuroidea) leads to revised morphological descriptions

Ophiuroidea is the largest class among extant echinoderms, with over 2000 described species assigned to 33 families. Here,  the first  identification key to  the recently revised classification was developed, and revised morphological descriptions were derived from it, expanding the previous short diagnoses. The key was built by analyzing internal and external skeletal characters of predominantly the type species of each family, including at least two mutually exclusive attributes per family. Various numeric and multistate characters were used to create a traditional as well as an interactive key using the DELTA and Xper software programs . Illustrations (SEM and digital photos) are included in the key to facilitate the assessment of character states by users. Not only is it the first identification key to the families, according to the recently proposed new classification and the examined species, but this interactive key also assists users in understanding the family level taxonomy of brittle stars. The interactive key allows new characters and states to be added, when more species will have been analyzed, without the need to reconfigure the complete key (as may be necessary with conventional keys).


Introduction
Ophiuroidea Gray, 1840 is one of the most diverse classes among extant echinoderms , currently including 2096 species (Stöhr et al. 2020). Ophiuroids are an ancient group of marine invertebrates with a fossil record dating back to the early Ordovician, about 480 ma (Shackleton 2005). They occupy marine habitats, ranging from the poles to the equator, from intertidal reefs to hadal depths of at least 6500 m (Smith et al. 1995;O'Hara et al. 2017). From the mid-eighteenth century, there has been a rise in the discovery rate of ophiuroid species, which has remained relatively high . Since the early-twentieth century, ophiuroid taxonomy has received more attention, as is evidenced by the production of traditional identifi cation keys for various geographical areas (Mortensen 1927;Clark & Rowe 1971;Paterson 1985;Smirnov et al. 2006;Pomory 2007;Martynov 2010a;Gondim et al. 2013), but all of these keys are based on a now-outdated classifi cation, which limits their use above the genus level. Matsumoto (1915Matsumoto ( , 1917 suggested the fi rst comprehensive classifi cation for the whole class, separating it into two subclasses, Myophiuroida Matsumoto, 1915, which possesses ventral arm plates that cover the ambulacral groove, and Oeogophiuroida Matsumoto, 1915, which lacks ventral arm plates. He divided the former into four diff erent orders, Phrynophiurina Matsumoto, 1915, Laemophuirina Matsumoto, 1915, Chilophiurina Matsumoto, 1915, Gnathophiurina Matsumoto, 1915 families. Matsumoto's (1915Matsumoto's ( , 1917 classifi cation was not universally accepted though, and Mortensen (1927) instead proposed two orders, Euryalae and Ophiurae, based on the articular structures of the vertebrae (streptospondylous vs zygospondylous). He also provided the fi rst traditional dichotomous identifi cation key for 12 families but it was restricted to the British Isles. On the other hand, Murakami (1963) agreed with Matsumoto (1915Matsumoto ( , 1917, and he identifi ed several new and taxonomically important characters from the examination of dental and oral plates. Fell (1960) included 17 families in his key to the ophiuroid genera, and while it is still useful, users need to be familiar with previous classifi cations to successfully navigate the key. All of the above-mentioned keys are conventional dichotomous keys. So far, only a single, taxonomically and geographically limited, computerized interactive key for Ophiuroidea is available (Alitto et al. 2018). Interactive keys have been created for many other taxa though, e.g, for Australian aquatic invertebrates using the software LUCID (Lucidcentral 2012), and also for various plants and animals using DELTA (Dallwitz 2019), but none includes echinoderms.
Eighty years after Matsumoto's (1915) work, Smith et al. (1995) proposed a new phylogeny, based on a cladistic analysis of all Ophiuroidea, including mostly morphological (43 characters) and limited molecular data, proposing 17 families. This study split the Ophiuroidea into the orders Euryalida Lamarck, 1816 and Ophiurida Müller & Troschel, 1840, based on vertebral articulation type, as Mortensen (1927) had done. Although the molecular study provided some support for the morphological tree, it left most of the deep branches unresolved (Smith et al. 1995) and was debated for methodological shortcomings (Stöhr 2012). Thanks to the studies on arm spine articulations (Martynov 2010b) and lateral arm plates (Thuy & Stöhr 2011), new phylogenetically informative characters were discovered, which led to a breakthrough with the successful inference of the ophiuroid phylogeny from morphological characters in great congruence with the molecular phylogeny (Thuy & Stöhr 2016), proposed two years earlier (O'Hara et al. 2014). At that time, the ophiuroid classifi cation comprised 19-20 families (Thuy & Stöhr 2016). Recently, O'Hara et al. (2017) presented a new molecular phylogeny, which resulted in a new and revised classifi cation that increased the number of families to 33 (O'Hara et al. 2018). The new phylogeny found several of the previous families to be paraphyletic or polyphyletic (O'Hara et al. 2017), which were revised in the new classifi cation, and many genera were transferred to other families than their traditional placement (O'Hara et al. 2018). Consequently, existing identifi cation keys to the previous families can no longer be used. One might argue that species identifi cation keys are more useful than family keys, but 1) these are lacking or outdated for the majority of species, 2) they are usually geographically limited, 3) identifi cation to high ranks is useful for an estimate of diversity and can speed up faunal inventory studies, and 4) the new classifi cation diff ers greatly from all previous ones and a key can make it more accessible to users. Therefore, the present study aims to develop a new identifi cation key for the currently accepted ophiuroid families (O'Hara et al. 2018), while at the same time expanding the short diagnoses provided by O'Hara et al. (2018) into detailed descriptions using previously rarely considered characters.
The family level taxa of Ophiuroidea are based on nodes of similar age (± 110 ma) on the recent phylogeny (O'Hara et al. 2017), but as ranks in a classifi cation they are of course artifi cial constructs by and for humans to conceptualize the diversity. During the evolution within a family clade, many changes happened: some genera / species lost the synapomorphies that support the monophyly of each node on the phylogenetic tree, others kept some of them as plesiomorphies, and most evolved novelties / autapomorphies that may or may not need to be included in the family description. Also, the within-family phylogeny has not been analyzed yet, apart from few exceptions (e.g., Ophiocomidae Ljungman, 1867;O'Hara et al. 2019), and the support for each family is biased towards molecular data. Previous descriptions of many families no longer refl ect the morphological diversity of the included genera, e.g., the family Ophiomyxidae Ljungman, 1867 now includes several genera that were previously assigned to Ophiodermatidae Ljungman, 1867, whereas several former ophiomyxids have been transferred to the resurrected Ophioscolecidae Lütken, 1869 (which is currently paraphyletic). The assemblage of genera included in Ophiomyxidae now share rather few characters. This complicates the identifi cation of family-specifi c characters that allow the correct familial assignment of all species by morphology, which was emphasized also by O'Hara et al. (2018), who limited the family descriptions to a few diagnostic characters based on the type genera. Pending full taxonomic revisions of all families, the present study aims to provide revised descriptions, based on the characters used for the creation of an interactive identifi cation key.
A conventional (often dichotomous) key has a single access point and is fi xed in the order in which the characters need to be assessed, and with an increasing number of steps, it quickly becomes unwieldy and imprecise, because the probability of making a wrong decision (and then following the wrong path) increases with the number of decisions to be made. Attempts have been made to better structure excessively long keys (with hundreds of couplets to evaluate), but they still have severe limitations for incomplete specimens and human error, problems that can be overcome with computerized multiple access keys (Walter & Winterton 2007). The main limitation when constructing any kind of key is defi ning the most suitable characters and states that accurately identify a specifi c taxon, which can be diffi cult when the taxa are not suffi ciently distinct from each other. To overcome the above-mentioned obstacles, we chose to create a software-based interactive key in which the user chooses / enters the character states of the specimen that can be observed, not limited by a particular order of decisions as in traditional keys, and thus off ering multiple access points. The program eliminates taxa to which the chosen character states do not apply, and the process is continued until only one taxon remains . This allows the inclusion of a large number of characters, and species (or incomplete specimens), which lack some of the typical features, can still be identifi ed. Redundancy also ensures that user errors have less impact on the outcome. It should be noted that the characters used in this key were chosen for family identifi cation purposes, not for phylogenetic inference and also not for identifi cation at lower taxonomic levels (genus, species). This means that homoplastic characters (e.g., madreporite shape, branching arms) can be useful for identifi cation when they can narrow down the selection of possible results, and autapomorphies are particularly useful for identifi cation but not for phylogenetic inference. Characters that vary between species (e.g., disc color, number of distal / proximal spines and tentacle scales) were excluded as not useful for the identifi cation of families. Consequently, the character matrix diff ers considerably from the one used to infer a phylogeny from morphological characters (Thuy & Stöhr 2011) and would not be suitable for that purpose.
Traditionally, ophiuroids have been described based on limited external structures, such as disc scales, arm plates and spines, and oral frame skeleton, among others. Modern descriptions also include microscopic structures, observed by scanning electron microscopy, and internal skeletal structures (Martynov 2010a; European Journal of Taxonomy 766: 1-63 (2021) 4 Stöhr 2012; Stöhr &Martynov 2016;Okanishi et al. 2017), and many other characters have been reevaluated (Thuy & Stöhr 2016, 2018. Despite this interest, the terminology used for ophiuroid skeletal elements is not standardized and authors have used diff erent terms, which might lead to confusion for readers. Also, the descriptions of the 33 families in the new classifi cation were limited to a few diagnostic characters (O'Hara et al. 2018). Therefore, besides creating the keys, we elaborated on some terminology and expanded the descriptions for each family. These descriptions cannot replace full taxonomic revisions, but since those are time-consuming and will probably not be achieved in the nearest future, the key-derived descriptions are a useful compromise, providing more information than the short diagnoses by O'Hara et al. (2018). To evaluate the diff erences between interactive and conventional keys, and to provide a tool that can be used without a computer, we used the software to export a conventional polychotomous key, which prioritizes external characters to facilitate usage without the need for extensive preparation of the animals.

Material and methods
Primarily, the type species of each family was used to fi nd the features that may distinguish the families. When the type species was not available, another species of the type genus was evaluated, and if no member of the type genus was available, other genera in the family were used. Additional species from most families were used to verify the presence of the chosen characters and to test the key. The species assessed in this study are shown in Table 1. Characters were collected by examining images in publications (Smith et al. 1995;Pomory 2007;Gondim et al. 2013), preserved specimens collected in the Persian Gulf and in the museum collections of the Royal Belgian Institute of Natural Sciences (RBINS), as well as examining scanning electron microscope images (SEM) and 3D images of museum specimens of the Swedish Museum of Natural History (SMNH) and RBINS. We used 54 external and 22 internal characters, amounting to a total of 77 characters and 190 states (Appendix 1), to create the interactive key. The characters and states were defi ned partially based on O'Hara et al. (2018), modifi ed for the current purpose, and additional characters were selected after specimen / image examination. All characters include at least two mutually exclusive states. The genera included in each family were extracted from O'Hara et al. (2018) and verifi ed for possible later changes in Stöhr et al. (2020).
The primary software package used to develop a traditional and interactive key was DELTA (DEscription Language for TAxonomy) (Dallwitz et al. 2006). It allows the choice of diff erent types of characters such as ordered, unordered, integer, real numeric and text, but we omitted real number characters, e.g., 'disc diameter' as an absolute size, because not only do the animals change size during growth, but also it might not be easy to measure the targeted character precisely while using the key. Instead, using a multiple-choice character with ranges of values or relative size, e.g., 'the arm, is × times as long as the disc diameter' is more useful. Selected images were uploaded and labeled in the software to enable users to compare to their specimens, to facilitate the choices to be made. To create a key, a great deal of attention must be paid to choose the most powerful characters and states for family discrimination. Regarding character dependency, DELTA has the capability to defi ne dependencies between characters, i.e., if the character 'A' is not present in a taxon, the other dependent characters describing character 'A' cannot be applied (see characters 42 and 43-45 in Appendix 1). On the other hand, missing data for some states were left blank and written as "no data" in the family description. These may be completed in the future as the project progresses and more morphological traits will be included. This is the main advantage of the interactive key, i.e., that it allows new characters to be added without disrupting the existing structure.
For the creation of the conventional key, of the original 77 characters, 46 characters were found to be suffi cient to separate the taxa. We prioritized external characters in the creation of the conventional key as much as possible (without compromising identifi cation accuracy), because these are more easily observed under fi eld conditions. To achieve this, DELTA off ers techniques such as weighting and setting reliability values. Weighting in DELTA is not related to weighting in phylogenetic inference. It serves Table 1 (continued on next page). List of the species assessed to create the identifi cation key to the ophiuroid families. The type species of each family is marked with a "T", square brackets [] indicate species that could not be assessed due to lack of material, and those with an asterisk (*) were assessed mainly from literature. Ophiolepis superba H.L. Clark, 1915 (T) Ophioleucidae Ophioleuce Koehler, 1904 Ophioleuce seminudum Koehler, 1904 (T) Ophiopallas paradoxa Koehler, 1904Ophiomusaidae Ophiomusa Hertz, 1927 Ophiomusa  (Koehler, 1904) Amphiophiura latro (Koehler, 1904) (Koehler, 1923) Ophiura sarsii Lütken, 1855 Ophiocten sericeum (Forbes, 1852) *citations: Clarkcomidae (Devaney 1970;Hendler 2018), Ophiobyrsidae (Martynov 2010a;O'Hara et al. 2018) and Ophiopezidae (Martynov 2010a). The type species of each family is marked with a "T", square brackets [] indicate species that could not be assessed due to lack of material, and those with an asterisk (*) were assessed mainly from literature.

European Journal of
the purpose of letting characters with high reliability be used more in the key compared to those with lower reliability. Reliable characters are those whose state values can easily be assigned to a specimen and thus most accurately separate the taxa. We used a higher reliability value (8) for external characters and a lower value (1) for internal and diffi cult characters in the creation of the conventional key. For the interactive key the reliability value was set as the default (5). We assigned weights to characters that are more reliable for identifi cation, which causes them to appear fi rst in the interactive key and increases their likelihood of being used in the creation of the conventional key. As an alternative to DELTA, the same dataset was transferred to Xper2 (desktop application, ver. 2.3.2) and Xper3 (online application), in which we applied character weights (= reliability) ranging between 1 and 5, to create conventional and interactive keys. The character reliability in DELTA for characters 1-77 was as follows: 1,

Terminology
Disc (fl at / high) Flat or high stands for the height of the disc relative to the level of the arms; if it is at about the same level as the arms, such as in Ophiura Lamarck, 1801, it is considered fl at; in e.g., Amphiophiura Matsumoto, 1915 it is considered as high. Sac-like refers to a soft, fl oppy disc that is much higher than wide, e.g., as in Ophiomyces Lyman, 1869, where it may be caused by the absence of the radial shields. See character 35 and Fig. 1A-C.
to half / less than half or none. To estimate the size of the scales, we considered the interradius as a triangle and counted the scales on the H line of the triangle. The state 'few scales with whole coverage' implies that the size of the scales is greater than with 'many scales and whole / half coverage'. See characters 47-49 and Fig. 2C Koehler, 1904). L. Ventral part of LAP not projecting ventro-proximalwards (Ophiosphalma fi mbriatum (Koehler, 1922)).
Oral papillae sensu lato (according to Hendler 2018) One angle of the oral frame includes a dental plate (DP), two oral plates (= half-jaws) (OP), two adoral shields (AdSh), and one oral shield (OSh), positioned from proximal towards distal. The terminology for the articulated ossicles situated at the jaws has been clarifi ed by Hendler (2018). Oral papillae sensu lato are small ossicles (papilliform, or spiniform) attached to the edges of the plates (DP, OP, AdSh, VAP-1, and vCtP) around the ophiuroid oral frame. We used the following papillae in the key. Those on the DP (described below) include teeth and tooth papillae. Infradental papillae are only the pair that develops later than others on the DP / OP joint and ultimately articulate with the OP, then move under the ventralmost tooth (situated on the most ventral part of the DP). Secondary infradental papillae (2IPas) appear close to the DP / OP joint. Each new ossicle arises proximal to its predecessor on the OP in the same direction. The adoral shield spine (AdShSp) is mainly positioned on the adoral shield (AdSh). However, the position of the AdShSp can vary during ontogenesis. It may remain on the AdSh (as in Ophioderma rubicundum Lütken, 1856), overlap the AdSh and the OP (as in Ophiolepis paucispina (Say, 1825)), or articulate with the OP (as in Ophionereis olivacea H.L. Clark, 1900). The secondary adoral shield spine (2AdShSp) is homologous with the AdShSp, and both are situated next to each other. The buccal scale, which is a plesiomorphy in Ophiuroidea, but might be homologous within several orders (secondarily absent in some taxa), is situated inside the oral slit and next to the oral plate ridge. Lateral oral papillae (LOPa) are all remaining ossicles not named above that develop on the OP distal to the IPa (Hendler 2018). See characters 50-58 and Figs 2E, 4.

Dental plate
The dental plate sits vertically inside the mouth, covering the proximal edges of two oral plates (halfjaws). We refer to the end closest to the upper / aboral disc as dorsal and the end at the mouth opening, situated below the teeth, as ventral. Since the terms aboral (away from the mouth) and oral (close to the mouth) may be less precise when talking about the mouthparts, ventral / dorsal have been preferred by numerous previous authors, e.g., Hendler (2018) and O'Hara et al. (2018), and could be considered the standard terminology in ophiuroid morphology. Dental sockets on the DP are either a depression or a perforation that may be with / without a septum. A foramen (pl. foramina) is a hole perforating the socket on the DP, and a perforation is a smaller hole than a foramen. Perforating holes are divided into two types, one with distinct borders and the other with indistinct borders (e.g., Ophiolepis superba H.L. Clark, 1915 andOphiactis savignyi (Müller &Troschel, 1842), respectively). The geometry of the DP (entire / fragmented) varies from equal width all over to ventral or dorsal half widest. To infer the size of dental sockets, we estimated whether their height is less than 50% of their width, equal or wider. Another character to describe the DP is the ratio of teeth to tooth papillae on the DP, i.e., whether there is only a single column of teeth, or with a single column on half of the DP and the other half with tooth papillae (e.g., Ophiotrichidae Ljungman, 1867, Ophiopholidae O'Hara, Stöhr, Hugall, Thuy & Martynov, 2018), or with a single column on ⅔ of the DP and tooth papillae on ⅓ of the DP (e.g., Ophiuridae Müller & Troschel, 1840), or covering the entire DP with teeth or tooth papillae evenly (e.g., Gorgonocephalidae Ljungman, 1867). See characters 68-74 and Fig. 3F-I.

Arm spine shape
Considering the 'tip' and 'cross-section' shape, as well as the 'ratio of distal ⅓ of an arm spine to the maximum proximal thickness' (in a given cross-sectional plane), four states are defi ned for this study (Fig. 5). Tapering / conical (distally less than half as thick as proximal diameter with round cross-section and tapering tip); fl at (distally half as thick as proximal diameter or same with oval cross-section); cylindrical (same thickness at both ends with fl at / round tip); pointed (distally half to equally as thick as proximal diameter with round cross-section and pointed tip). See characters 12-17 and Fig. 5.

Tentacle scales
TSc develop later than arm spines and are situated next to the tube foot (= tentacle) pore, often at the LAP, sometimes on the VAP or both. The spines have visible articulations on the LAP, while the TSc has no articulation (except in Ophiomyces). The pores may be visible along the arm or only on the proximal part (e.g., in Ophiomusa Hertz, 1927 only the fi rst two are visible, while in Ophiosphalma H.L. Clark, 1941 the pores are visible on more than two proximal segments, and in most other genera they are present along the entire arm). We also described the tentacle scale length (compared with the nearest spine) and shape (comparing the length with TSc width) accompanied by the oval / round estimated cross-section. See characters 18-22.  One angle of the oral frame includes a dental plate (DP), two oral plates (= half-jaws) (OP), two adoral shields (AdSh), and one oral shield (OSh), positioned from proximal towards distal. The typical position of teeth, tooth papillae, secondary infradental papillae (2IPa), infradental papillae (IPa), buccal scale, secondary adoral shield spine (2AdShSp), adoral shield spine (AdShSp) on DP, OP and AdSh are illustrated, respectively. The presence / absence, number and shape of each papilla vary among diff erent taxa. : distally ⅓ as thick as proximal diameter with a round cross-section; Pointed: distally half as thick as proximal diameter with a round cross-section; Cylindrical: same thickness at both ends with fl at/round tip. B. Flat: distally half as thick as proximal diameter with oval cross-section. Red dashed lines show the ⅓ distal cross-section. The letters ʻaʼ and ʻbʼ illustrate the diameter of ⅓ distal and basal cross-sections, respectively. ʻa/bʼ shows the approximate ratio of these diameters.

Keel
A keel is a longitudinal, median and often raised structure on the dorso-distal part of the vertebrae. The vertebral keel was originally introduced by LeClair (1996) as the structure projecting distally from the dorsal groove of the vertebrae, extending beyond the length of the articulating structures and matched by a large depression (socket) on the proximal face of the adjacent vertebra. It is only found in Ophiotrichidae, Ophionereididae Ljungman, 1867 and Ophiopholidae. This defi nition was applied by most later authors (LeClair & LaBarbera 1997;O'Hara et al. 2018;), whereas Martynov et al. (2015) used a broader defi nition, including short structures protruding from the dorso-distal part of the vertebrae in Ophiacanthidae Ljungman, 1867 and forming an arrow-shaped structure, but not extending beyond the articulating structures and not matched by a proximal deep socket. The latter broader defi nition reduces the taxonomic value of the keel and is rejected here. To avoid misunderstandings, we modifi ed the term to 'extended keel', meaning the original defi nition of a long distal process with matching proximal socket. See character 76 and Fig. 3C-D.

Results
This study resulted in traditional and interactive keys for the families of Ophiuroidea, based on the classifi cation proposed by O'Hara et al. (2018) and the examined species listed in Table 1. The key is based on a limited number of species and only on adult specimens, which may limit its usefulness for other species and juvenile stages. Although we assigned highest and lowest weight to the same characters in DELTA and Xper, the resulting keys were diff erent. Giving lower scores to some characters increased the length of the conventional key on both platforms. The interactive keys are available as a DELTA fi le package (https://zenodo.org/record/4563203) and as an Xper3 online application (http://xper3.fr/xper3GeneratedFiles/publish/identifi cation/7495441984259574364/). Below we present the conventional key created by DELTA. In Appendix 2, a condensed tabular key is available; its full text version is provided as Supp. fi le 1. The full character and item matrix (tabular key) is provided as Supp. fi le 2. The conventional key produced by Xper 3 is available as Supp. fi le 3. A comparison of both software programs and their features is provided in Supp. fi le 4.
Below, all 33 families are described as we understand them by the examined species. The descriptions were generated with the aid of DELTA Intkey  and edited manually.

Interactive Key in DELTA format
We recommend the usage of the CSIRO version of DELTA Intkey for better performance and compatibility. When the interactive key is running, the characters are listed on the left (it is also possible to ask the software to show all or the best characters to separate the taxa). In addition, on the upper right, all taxa, and on the lower half, only eliminated ones are shown (Fig. 6). By clicking on a character, the states will be shown to choose from and images can be requested to distinguish the states precisely. After selecting the states, the software eliminates some taxa. The remaining taxa are then shown in the upper right half of the window. The user can continue the identifi cation process based on the characters that are possible to observe on the specimen and narrow the identifi cation until one taxon remains. Moreover, characters can be used in any sequence. Some family characters may not be present or ambiguous in some species. Therefore, it is possible to skip ahead, facilitating faster identifi cation.
European Journal of Taxonomy 766: 1-63 (2021) 14 Fig. 6. DELTA interactive key workspace. The software window shows: available characters (blue box), used characters (grey), remaining taxa (green) and eliminated taxa (red). 9(8). Arm spines predominantly parallel to arm axis (adpressed); integument obscuring plates (Fig. 1D); dorsal disc without scales; dorsal disc with spines ( Fig 2B, D) (21). Arm combs present (Fig. 1F); arm spines predominantly parallel to arm axis (adpressed); tentacle scales slightly longer than wide; ventral teeth several (tooth papillae) (Fig. 4)  The number of scales on H line ( Fig. 2C) of an interradius about 14-21; arm spine articulation ventralwards increasing in size; dental plate dorsal half widest; dental sockets equal to or more than 50% of the width ..   Table 1 In total, there are 33 characters for the arm, 29 for the ventral part of the disc, including the oral frame, 12 for the dorsal disc, and three characters for vertebrae. The language is software generated, slightly human-modifi ed.

Description
A . Arm shape unbranched; length more than 4 × disc diameter; covering without granules / spines / tubercles. Arm combs absent. Dorsal arm plate absent. Accessory dorsal arm plate absent. Ventral arm plate present. Accessory ventral arm plate absent. Girdle of tiny hooks on the arms absent. Spines not all similar in shape ( Fig. 2F-H). The atypical spines are hook-shaped (Fig. 2G). Arm spine cross section round. The ratio of thickness of distal ⅓ of arm spine to maximum thickness (in a given cross-sectional plane) 0.2-0.6. Arm spines tapering (conical); at proximal segments only on ventral side of arms; predominantly erect, standing perpendicular to arm axis. Longest arm spines shorter than half a segment. Tentacle scales absent. Position of lateral arm plate (LAP) only lateral ( Fig. 3J-K). Dorsal and / or ventral edge(s) of LAPs without constriction. Ventral position of LAP not projecting ventro-proximalwards. Ventro-distal tip of LAP not projecting ventralwards (Fig. 2L). Arm spine articulation without dorsal and ventral lobes; arranged over entire distal LAP edge; all similar. Distance between spine articulations ventralwards increasing. Nerve and muscle openings separated by large, prominent ridge or regular stereom. Nerve opening smaller than muscle opening.

D
. Disc pentagonal to circular; high (above arm level). Integument obscuring plates. Dorsal disc without granules; with concealed primary plates; without scales (A. loveni Müller & Troschel, 1842) or a few (O'Hara et al. 2018); without spines. Dorsal disc scale / plate without tubercles. Radial shield present; without granules / spines; length more than half of the disc radius; bar-like.

V
. Gonads restricted to the disc. Ventral interradii without granules / spines. Scales on ventral interradii absent. Ventral teeth several (tooth papillae). Infradental papillae on oral plate absent. Buccal scales on OP absent. Accessory oral papillae on OP present. Lateral oral papillae on OP absent. Adoral shield spine absent. Lyman's ossicle absent. Oral shield covering less than one third of interradius; about as long as wide. Madreporite one; with numerous pores. Adoral shield meeting in front of the oral shield. Abradial genital plate not twisted. Number of genital slits one per bursa. Genital slit shorter than half interradius. Genital papillae absent. Dental plate of equal width all over; entire (single plate). Dental sockets slit-shaped to circular; less than 50% of the width. At least one socket on DP as depression or perforation without septum. Teeth spine-shaped. Teeth on DP arranged as covering the entire dental plate evenly.

Description
A . Arm shape branched; length more than 4 × disc diameter; covering with granules. Arm combs absent. Dorsal arm plate absent. Accessory dorsal arm plate absent. Ventral arm plate present or absent. Accessory ventral arm plate absent. Girdle of tiny hooks on the arms absent. Spines not all similar in shape. Atypical arm spines are hook-shaped. Arm spine cross section round. The ratio of thickness of distal ⅓ of arm spine to maximum thickness (in a given cross-sectional plane) 0.9-1. Arm spines cylindrical; at proximal segments only on ventral side of arms; predominantly erect, standing perpendicular to arm axis. Longest arm spines shorter than half a segment. Tentacle scales present; pores visible along the arm; nearly as long as wide; shorter than the spine nearby; cross section round. Position of lateral arm plate (LAP) only lateral. Dorsal and / or ventral edge(s) of LAPs without constriction (Fig. 2J). Ventral position of LAP not projecting ventro-proximalwards. Ventro-distal tip of LAP not projecting ventralwards. Arm spine articulation without dorsal and ventral lobes; restricted to (the ventral or central) portion of the distal edge; all similar. Distance between spine articulations not applicable (two spine articulation). Nerve and muscle openings separated by large, prominent ridge or regular stereom. Nerve opening approximately as large as muscle opening (Fig. 3A-B).

D
. Disc pentagonal; fl at (about same level as arms). Integument not obscuring plates. Dorsal disc with granules; with concealed primary plates; without scales; without spines. Dorsal disc scale / plate without tubercles. Radial shield present; with tubercles grown out of plate; length more than half of the disc radius; bar-like.

V
. Gonads not restricted to the disc (extending into the basal portion of the arms) or restricted to the disc (Asteroschema). Ventral interradii without granules / spines. Scales on ventral interradii absent. Ventral teeth single. Infradental papillae on oral plate present. Buccal scales on OP absent. Accessory oral papillae on OP absent. Lateral oral papillae on OP absent. Adoral shield spine absent. Lyman's ossicle absent. Oral shield covering less than one third of interradius; about as long as wide. Madreporite multiple; with one pore, or with numerous pores. Adoral shield meeting in front of the oral shield. Abradial genital plate not twisted. Number of genital slits one per bursa. Genital slit shorter than half interradius. Genital papillae absent. Dental plate of equal width all over; fragmented. Dental sockets slit-shaped; equal to or more than 50% of the width. At least one socket on DP as depression or perforation without septum. Teeth fl at (with round or slightly pointed tip, but never spine-like). Teeth on DP arranged as single row.

Description
A . Arm shape unbranched, or shape branched; length more than 4 × disc diameter; covering with granules. Arm combs absent. Dorsal arm plate present. Accessory dorsal arm plate absent. Ventral arm plate present. Accessory ventral arm plate absent. Girdle of tiny hooks on the arms present (Gorgonocephalus caputmedusae (Linnaeus, 1758)) or absent in some genera. Spines all similar in shape. Arm spine cross section oval. The ratio of thickness of distal ⅓ of arm spine to maximum thickness (in a given crosssectional plane) 0.9-1. Arm spines fl at; at proximal segments only on ventral side of arms; predominantly erect, standing perpendicular to arm axis. Longest arm spines between half and one segment long. Tentacle scales present; pores visible along the arm; slightly longer than wide; shorter than the spine nearby; cross section round. Position of lateral arm plate (LAP) only lateral. Dorsal and / or ventral edge(s) of LAPs without constriction. Ventral position of LAP projecting ventro-proximalwards. Ventro-distal tip of LAP not projecting ventralwards. Arm spine articulation without dorsal and ventral lobes; restricted to (the ventral or central) portion of the distal edge; all similar. Distance between spine articulations ventralwards increasing. Nerve and muscle openings separated by large, prominent ridge or regular stereom. Nerve opening smaller than muscle opening.

D
. Disc circular; fl at (about same level as arms). Integument obscuring plates. Dorsal disc with granules; with concealed primary plates; with scales; without spines. Dorsal disc scale / plate without tubercles. Radial shield present; with / without granules; length more than half of the disc radius; bar-like.

V
. Gonads restricted to the disc. Ventral interradii with granules. Scales on ventral interradii absent. Ventral teeth several (tooth papillae). Infradental papillae on oral plate absent. Buccal scales on OP absent. Accessory oral papillae on OP present. Lateral oral papillae on OP absent. Adoral shield spine absent. Lyman's ossicle absent. Oral shield covering less than one third of interradius; much wider than long. Madreporite one; with numerous pores. Adoral shield meeting in front of the oral shield. Abradial genital plate not twisted. Number of genital slits one per bursa. Genital slit shorter than half interradius. Genital papillae absent. Dental plate of equal width all over; fragmented. Dental sockets circular; equal to or more than 50% of the width. At least one socket on DP as depression or perforation without septum. Teeth spine-shaped. Teeth on DP arranged as covering the entire dental plate evenly.

Description
A . Arm shape unbranched; length 3-4 × disc diameter; covering with spines distal to the genital slit around the disc margin but without granules / spines / tubercles. Arm combs absent. Dorsal arm plate present. Accessory dorsal arm plate absent. Ventral arm plate present. Accessory ventral arm plate absent. Girdle of tiny hooks on the arms absent. Spines all similar. Arm spine cross section round. The ratio of thickness of distal ⅓ of arm spine to maximum thickness (in a given cross-sectional plane) 0.6-0.9. Arm spines pointed; mainly lateral; predominantly parallel to arm axis (adpressed). Longest arm spines shorter than half a segment. Tentacle scales present; pores visible only on proximal arm; nearly as long as wide; shorter than the spine nearby; cross section round. Position of lateral arm plate (LAP) arched (wrapped around the arm) (Fig. 3L). Dorsal and / or ventral edge(s) of LAPs without constriction. Ventral position of LAP not projecting ventro-proximalwards. Ventro-distal tip of LAP not projecting ventralwards. Arm spine articulation without dorsal and ventral lobes; arranged over entire distal LAP edge; ventralwards increasing in size. Distance between spine articulations dorsalwards increasing. Nerve and muscle openings separated by large, prominent ridge or regular stereom. Nerve opening approximately as large as muscle opening. D . Disc pentagonal; fl at (about same level as arms). Integument not obscuring plates. Dorsal disc without granules; with obvious primary plates; with scales; without spines. Dorsal disc scale / plate without tubercles. Radial shield present; without granules / spines; length more than half of the disc radius; scalene (oblique) triangular.

V
. Gonads restricted to the disc. Ventral interradii without granules / spines. Scales on ventral interradii covering the whole area of an interradius. The number of scales on H line of an interradius fewer than 7. Ventral teeth several (tooth papillae). Infradental papillae on oral plate present. Buccal scales on OP present. Accessory oral papillae on OP absent. Lateral oral papillae on OP absent. Adoral shield spine present. Lyman's ossicle absent. Oral shield covering less than one third of interradius; much longer than wide. Madreporite no data. Adoral shield meeting in front of the oral shield. Abradial genital plate not twisted. Number of genital slits one per bursa. Genital slit longer than half the length of an interradius. Genital papillae present. Dental plate of equal width all over; entire (single plate). Dental sockets slit-shaped; equal to or more than 50% of the width. At least one socket on DP as depression or perforation without septum. Teeth fl at (with round or slightly pointed tip, but never spine-like). Teeth on DP arranged as single row.

V
. Vertebrae zygospondylous; beyond segment 5 with short keel; without oral bridge. D . Disc pentagonal; high (above arm level). Integument not obscuring plates. Dorsal disc without granules; with obvious primary plates; with scales; without spines. Dorsal disc scale / plate with tubercles. Radial shield present; with tubercles, without granules / spines; length more than half of the disc radius; isosceles (mirror-symmetric) triangular to pear-shaped.

V
. Gonads restricted to the disc. Ventral interradii without granules / spines. Scales on ventral interradii covering the whole area of an interradius. The number of scales on H line of an interradius fewer than 7. Ventral teeth single. Infradental papillae on oral plate present. Buccal scales on OP present. Accessory oral papillae on OP absent. Lateral oral papillae on OP absent. Adoral shield spine present. Lyman's ossicle absent. Oral shield covering more than one third of the length of an interradius; much longer than wide. Madreporite no data. Adoral shield meeting in front of the oral shield. Abradial genital plate not twisted. Number of genital slits one per bursa. Genital slit shorter than half interradius. Genital papillae absent. Dental plate dorsal half widest; entire (single plate). Dental sockets slit-shaped and circular; equal to or more than 50% of the width. At least one socket on DP as depression or perforation without septum. Teeth fl at (with round or slightly pointed tip, but never spine-like). Teeth on DP arranged as single row.

V
. Vertebrae zygospondylous; beyond segment 5 with short keel; without oral bridge. . Gonads restricted to the disc. Ventral interradii without granules / spines. Scales on ventral interradii absent. Ventral teeth several (tooth papillae). Infradental papillae on oral plate present. Buccal scales on OP present. Accessory oral papillae on OP absent. Lateral oral papillae on OP present; single row of lateral papillae; block-shaped and spiniform. Adoral shield spine present. Lyman's ossicle absent. Oral shield covering less than one third of interradius; much wider than long. Madreporite no data. Adoral shield meeting in front of the oral shield. Abradial genital plate not twisted. Number of genital slits one per bursa. Genital slit longer than half the length of an interradius. Genital papillae absent. Dental plate and teeth no data.

V
. Vertebrae zygospondylous; beyond segment 5 with short keel; without oral bridge.

Description
A . Arm shape unbranched; length 3 × disc diameter; covering without granules / spines / tubercles. Arm combs present. Dorsal arm plate present. Accessory dorsal arm plate absent. Ventral arm plate present. Accessory ventral arm plate absent. Girdle of tiny hooks on the arms absent. Spines all similar in shape. Arm spine cross section round. The ratio of thickness of distal ⅓ of arm spine to maximum thickness (in a given cross-sectional plane) 0.6-0.9. Arm spines pointed; mainly lateral and at proximal segments only on ventral side of arms; predominantly parallel to arm axis (adpressed). Longest arm spines between half and one segment long, or between one and two segments long (Ophiura sarsii Lütken, 1855). Tentacle scales present or absent; pores visible along most of the arm; slightly longer than wide; shorter than the spine nearby; cross section oval. Position of lateral arm plate ( Integument not obscuring plates. Dorsal disc without granules; with obvious primary plates; with scales; without spines. Dorsal disc scale / plate without tubercles. Radial shield present; without granules / spines; length between one third and half of the disc radius; isosceles (mirror-symmetric) triangular to pear-shaped.

V
. Gonads restricted to the disc. Ventral interradii without granules / spines. Scales on ventral interradii covering the whole area of an interradius. The number of scales on H line of an interradius fewer than 7, or 7-14 (Ophiocten). Ventral teeth several (tooth papillae). Infradental papillae on oral plate present. Buccal scales on OP present. Accessory oral papillae on OP absent. Lateral oral papillae on OP absent. Adoral shield spine present. Lyman's ossicle absent. Oral shield covering more than one third of the length of an interradius; much longer than wide or about as long as wide (Ophiocten). Madreporite one; with one pore or with numerous pores. Adoral shield meeting in front of the oral shield. Abradial genital plate not twisted. Number of genital slits one per bursa. Genital slit longer than half the length of an interradius. Genital papillae present. Dental plate of equal width all over, or ventral half widest (Ophiura sarsii, Ophiocten); entire (single plate). Dental sockets circular; equal to or more than 50% of the width. At least one socket on DP as depression or perforation without septum. Teeth fl at (with round or slightly pointed tip, but never spine-like). Teeth on DP arranged as a single row on ⅔ and a cluster of tooth papillae on ⅓ of dental plate (two rows in Ophiocten).

V
. Vertebrae zygospondylous; beyond segment 5 with short keel; without oral bridge.

Description
A . Arm shape unbranched; length 3 × disc diameter; covering without granules / spines / tubercles. Arm combs present. Dorsal arm plate present. Accessory dorsal arm plate absent. Ventral arm plate present. Accessory ventral arm plate absent. Girdle of tiny hooks on the arms absent. Spines all similar in shape. Arm spine cross section round (Amphiophiura, Ophioplinthus), or oval (Ophiosparte). The ratio of distal 1 / 3 an arm spine to maximum thickness (in a given cross-sectional plane) 0.2-0.6, or 0.6-0.9 (Amphiophiura), or 0.9-1 (Ophiosparte, Ophioplinthus). Arm spines tapering (conical), or cylindrical (Ophiosparte), or pointed; mainly lateral; predominantly parallel to arm axis (adpressed). Longest arm spines shorter than half a segment (Amphiophiura, Ophioplinthus, Ophiopleura), or between half and one segment (Ophiosparte). Tentacle scales present; pores visible along most of the arm; more than two times as long as wide; shorter than the spine nearby; cross section oval. D . Disc circular; high (above arm level). Integument not obscuring plates (Amphiophiura, Ophiopyrgus, Ophioplinthus, Ophiopleura), or skin obscuring plates (Ophiosparte). Dorsal disc without granules; with obvious primary plates (Amphiophiura, Ophioplinthus), or with concealed primary plates (Ophiosparte, Ophiopleura); with scales (the scales are covered by thickened skin in Ophiosparte and embedded in thickened skin in Ophiopleura) without spines. Dorsal disc scale / plate without tubercles. Radial shield present; without granules / spines; length less than one third of the disc radius, or length between one third and half of the disc radius (Ophiopyrgus), or length more than half of the disc radius (Ophiopleura); isosceles (mirror-symmetric) triangular to pear-shaped.

V
. Gonads restricted to the disc. Ventral interradii without granules / spines. Scales on ventral interradii covering the whole area of an interradius, or absent (Ophiosparte, Ophiosparte, Ophiopleura). The number of scales on H line of an interradius about less than 7. Ventral teeth several (tooth papillae). Infradental papillae on oral plate present. Buccal scales on OP present. Accessory oral papillae on OP present. Lateral oral papillae on OP absent. Adoral shield spine present. Lyman's ossicle absent. Oral shield covering less than one third of interradius; much longer than wide or about as long as wide (e.g., Ophioplinthus tessellata (Verrill, 1894) and Ophiopleura borealis Danielssen & Koren, 1877). Madreporite no data. Adoral shield meeting in front of the oral shield. Abradial genital plate not twisted. Number of genital slits one per bursa. Genital slit longer than half the length of an interradius (or about equal). Genital papillae present. Dental plate ventral half widest; entire (single plate). Dental sockets circular; less than 50% of the width. At least one socket on DP as depression or perforation without septum. Teeth spine-shaped. Teeth on DP arranged as a single row on ⅔ and a cluster of tooth papillae on ⅓ of dental plate (Ophioplinthus).

V
. Vertebrae zygospondylous; beyond segment 5 with short keel; without oral bridge.  (Fig. 2F). Arm spine cross section round (Ophiotholia), or oval (Ophiomyces). The ratio of thickness of distal ⅓ of arm spine to maximum thickness (in a given cross-sectional plane) 0.2-0.6, or 0.9-1. Arm spines tapering (conical), or fl at (Ophiomyces), or cylindrical (Ophiotholia); mainly lateral; predominantly parallel to arm axis (adpressed). Longest arm spines between one and two segments long. Tentacle scales present; pores visible along most of the arm; slightly longer than wide; shorter than the spine nearby. Position of lateral arm plate (LAP) arched (wrapped around the arm). Dorsal and / or ventral edge(s) of LAPs with constriction (Ophiotholia), or without constriction (Ophiomyces). Ventral position of LAP projecting ventro-proximalwards. Ventro-distal tip of LAP projecting ventralwards (Fig. 2K). Arm spine articulation with dorsal and ventral lobes; without sigmoidal fold; arranged over entire distal LAP edge; ventralwards increasing in size. Distance between spine articulations equal. Nerve and muscle openings separated by small ridge if at all. Nerve opening smaller than muscle opening. D . Disc circular; a sac-like shape (much higher than wide). Integument obscuring plates. Dorsal disc without granules; with concealed primary plates; with scales; with spines. Dorsal disc scale / plate without tubercles. Radial shield absent.

V
. Gonads restricted to the disc. Ventral interradii without granules / spines (Ophiotholia), or with spines (Ophiomyces). Scales on ventral interradii absent. Ventral teeth several (tooth papillae). Infradental papillae on oral plate present. Buccal scales on OP absent. Accessory oral papillae on OP absent. Lateral oral papillae on OP present; multiple rows of lateral papillae; paddle-shaped. Adoral shield spine absent. Lyman's ossicle absent. Oral shield covering less than one third of interradius; much longer than wide. Madreporite no data. Adoral shield separated. Abradial genital plate not twisted. Number of genital slits one per bursa. Genital slit longer than half the length of an interradius. Genital papillae absent. Dental plate ventral half widest; entire (single plate). Dental sockets circular; less than 50% of the width. At least one socket on DP as depression or perforation without septum. Teeth fl at (with round or slightly pointed tip, but never spine-like). Teeth on DP arranged as a single row on ⅔ and a cluster of tooth papillae on ⅓ of dental plate.

V
. Vertebrae zygospondylous; beyond segment 5 with short keel; without oral bridge.

Description
A . Arm shape unbranched; length 3-4 × disc diameter; covering without granules / spines / tubercles. Arm combs absent. Dorsal arm plate present. Accessory dorsal arm plate absent. Ventral arm plate present. Accessory ventral arm plate absent. Girdle of tiny hooks on the arms absent. Spines all similar, or not all similar in shape (Ophiolycus). The atypical spines are hook-shaped. Arm spine cross section round. The ratio of thickness of distal ⅓ of arm spine to maximum thickness (in a given cross-sectional plane) 0.2-0.6, or 0.9-1 (Ophiolycus). Arm spines cylindrical (Ophiolycus) or tapering (conical); mainly lateral; predominantly erect, standing perpendicular to arm axis. Longest arm spines between one and two segments long. Tentacle scales present (Ophiolycus) or absent; pores visible along most of the arm; more than two times as long as wide; shorter than the spine nearby; cross section oval. Position of lateral arm plate (LAP) arched (wrapped around the arm). Dorsal and / or ventral edge(s) of LAPs with constriction. Ventral position of LAP projecting ventro-proximalwards. Ventro-distal tip of LAP projecting ventralwards. Arm spine articulation with dorsal and ventral lobes; without sigmoidal fold; arranged over entire distal LAP edge; dorsalwards increasing in size. Distance between spine articulations dorsalwards increasing. Nerve and muscle openings separated by small ridge if at all. Nerve opening smaller than muscle opening. D . Disc circular; fl at (about same level as arms). Integument obscuring plates. Dorsal disc without granules; with concealed primary plates; with or without scales; without spines. However, in some species with granules and spines (O'Hara et al. 2018). Dorsal disc scale / plate without tubercles. Radial shield absent.

V
. Gonads restricted to the disc. Ventral interradii without granules / spines. Scales on ventral interradii covering the whole area of an interradius. The number of scales on H line of an interradius about 14-21. Ventral teeth several (tooth papillae). Infradental papillae on oral plate present. Buccal scales on OP absent. Accessory oral papillae on OP present. Lateral oral papillae on OP absent. Adoral shield spine present. Lyman's ossicle absent. Oral shield covering less than one third of interradius; much wider than long. Madreporite one or multiple; with one pore or with numerous pores. Adoral shield meeting in front of the oral shield. Abradial genital plate not twisted. Number of genital slits one per bursa. Genital slit longer than half the length of an interradius. Genital papillae absent. Dental plate ventral half widest; entire (single plate). Dental sockets circular; equal to or more than 50% of the width. At least one socket on DP as depression or perforation without septum. Teeth spine-shaped. Teeth on DP arranged as a single row on half of dental plate and other half with a cluster of tooth papillae (ventral cluster), or a single row on ⅔ and a cluster of tooth papillae on ⅓ of dental plate (Ophiolycus).

Description
A . Arm shape unbranched; length more than 4 × disc diameter; covering without granules / spines / tubercles. Arm combs absent. Dorsal arm plate present. Accessory dorsal arm plate absent. Ventral arm plate present. Accessory ventral arm plate absent. Girdle of tiny hooks on the arms absent. Spines all similar in shape. Arm spine cross section round. The ratio of thickness of distal ⅓ of arm spine to maximum thickness (in a given cross-sectional plane) 0.6-0.9. Arm spines pointed; mainly lateral; predominantly erect, standing perpendicular to arm axis. Longest arm spines between half and one segment long. Tentacle scales absent. Position of lateral arm plate (LAP) arched (wrapped around the arm). Dorsal and / or ventral edge(s) of LAPs with constriction. Ventral position of LAP projecting ventro-proximalwards. Ventro-distal tip of LAP projecting ventralwards. Arm spine articulation with dorsal and ventral lobes; without sigmoidal fold; arranged over entire distal LAP edge; ventralwards increasing in size. Distance between spine articulations equal. Nerve and muscle openings separated by large, prominent ridge or regular stereom. Nerve opening smaller than muscle opening.

D
. Disc circular; fl at (about same level as arms). Integument obscuring plates. Dorsal disc with granules; with concealed primary plates; without scales; with spines. Dorsal disc scale / plate without tubercles. Radial shield present; with granules.

V
. Gonads restricted to the disc. Ventral interradii without granules / spines. Scales on ventral interradii absent. Ventral teeth several (tooth papillae). Infradental papillae on oral plate present. Buccal scales on OP absent. Accessory oral papillae on OP absent. Lateral oral papillae on OP present. Adoral shield spine present. Lyman's ossicle present. Oral shield covering less than one third of interradius; about as long as wide. Madreporite no data. Adoral shield meeting in front of the oral shield. Abradial genital plate not twisted. Number of genital slits one per bursa. Genital slit longer than half the length of an interradius. Genital papillae absent. Dental plate of equal width all over; entire (single plate). Dental sockets circular; less than 50% of the width. At least one socket on DP as depression or perforation without septum. Teeth spine-shaped. Teeth on DP arranged as covering the entire dental plate evenly.

Type genus
Ophiocamax Lyman, 1878. (No other genera) Description A . Arm shape unbranched; length more than 4 × disc diameter; covering tubercles grown out of scales / plates. Arm combs absent. Dorsal arm plate present. Accessory dorsal arm plate absent. Ventral arm plate present. Accessory ventral arm plate absent. Girdle of tiny hooks on the arms absent. Spines all similar in shape. Arm spine cross section round. The ratio of thickness of distal ⅓ of arm spine to maximum thickness (in a given cross-sectional plane) 0.9-1. Arm spines cylindrical; mainly lateral; predominantly erect, standing perpendicular to arm axis. Longest arm spines longer than two segments. Tentacle scales present; pores visible along most of the arm; slightly longer than wide; shorter than the spine nearby; cross section oval. Position of lateral arm plate (LAP) only lateral. Dorsal and / or ventral edge(s) of LAPs without constriction. Ventral position of LAP projecting ventro-proximalwards. Ventrodistal tip of LAP projecting ventralwards. Arm spine articulation with dorsal and ventral lobes; without sigmoidal fold; arranged over entire distal LAP edge; dorsalwards increasing in size. Distance between spine articulations dorsalwards increasing. Nerve and muscle openings separated by small ridge if at all. Nerve opening smaller than muscle opening.

D
. Disc circular; fl at (about same level as arms). Integument not obscuring plates. Dorsal disc without granules; with concealed primary plates; with scales; with spines. Dorsal disc scale / plate without tubercles. Radial shield present; without granules / spines; length between one third and half of the disc radius; scalene (oblique) triangular.

V
. Gonads restricted to the disc. Ventral interradii with spines. Scales on ventral interradii absent. Ventral teeth several (tooth papillae). Infradental papillae on oral plate present. Buccal scales on OP absent. Accessory oral papillae on OP absent. Lateral oral papillae on OP present; multiple rows of lateral papillae; spiniform. Adoral shield spine present. Lyman's ossicle present. Oral shield covering more than one third of the length of an interradius; much wider than long. Madreporite one; with numerous pores. Adoral shield meeting in front of the oral shield. Abradial genital plate not twisted. Number of genital slits one per bursa. Genital slit longer than half the length of an interradius. Genital papillae absent. Dental plate of equal width all over; entire (single plate). Dental sockets slit-shaped and circular; equal to or more than 50% of the width. At least one socket on DP as depression or perforation without septum. Teeth spine-shaped. Teeth on DP arranged as a single row on ⅔ and a cluster of tooth papillae on ⅓ of dental plate.

Description
A . Arm shape unbranched; length 3-4 × disc diameter; covering without granules / spines / tubercles. Arm combs absent. Dorsal arm plate present. Accessory dorsal arm plate absent. Ventral arm plate present. Accessory ventral arm plate absent. Girdle of tiny hooks on the arms absent. Spines all similar in shape. Arm spine cross section oval. The ratio of thickness of distal ⅓ of arm spine to maximum thickness (in a given cross-sectional plane) 0.6-0.9 to 0.9-1 (in Ophiocopa more pointed than in Ophiotreta). Arm spines fl at; mainly lateral; predominantly erect, standing perpendicular to arm axis. Longest arm spines between one and two segments long, or longer than two segments (Ophiocopa). Tentacle scales present; pores visible along most of the arm; nearly as long as wide (Ophiotreta), or slightly longer than wide (Ophiocomina); shorter than the spine nearby; cross section oval. Position of lateral arm plate (LAP) arched (wrapped around the arm). Dorsal and / or ventral edge(s) of LAPs without constriction. Ventral position of LAP not projecting ventro-proximalwards. Ventro-distal tip of LAP not projecting ventralwards. Arm spine articulation with dorsal and ventral lobes; with a fully developed sigmoidal fold; arranged over entire distal LAP edge; dorsalwards increasing in size. Distance between spine articulations dorsalwards increasing. Nerve and muscle openings separated by small ridge if at all. Nerve opening smaller than muscle opening.

D
. Disc circular; high (above arm level). Integument obscuring plates. Dorsal disc with granules; with concealed primary plates; with scales; with spines. Dorsal disc scale / plate without tubercles. Radial shield present; with granules; length less than one third of the disc radius; scalene (oblique) triangular.

V
. Gonads restricted to the disc. Ventral interradii with granules. Scales on ventral interradii covering the whole area of an interradius. The number of scales on H line of an interradius more than 21. Ventral teeth several (tooth papillae). Infradental papillae on oral plate present. Buccal scales on OP absent. Accessory oral papillae on OP absent. Lateral oral papillae on OP present; single row of lateral papillae; rounded (Ophiotreta) or spiniform (Ophiocomina). Adoral shield spine present. Lyman's ossicle present. Oral shield covering less than one third of interradius; about as long as wide (Ophiotreta), or much wider than long (Ophiocomina). Madreporite one; with one pore. Adoral shield meeting in front of the oral shield. Abradial genital plate not twisted. Number of genital slits one per bursa. Genital slit longer than half the length of an interradius. Genital papillae absent. Dental plate ventral half widest; entire (Ophiocomina) or fragmented (Ophiotreta, Ophiocopa). Dental sockets slit-shaped and circular; less than 50% of the width or equal to or more than 50% of the width. At least one socket on DP as depression or perforation without septum. Teeth spine-shaped. Teeth on DP arranged as a single row on ⅔ and a cluster of tooth papillae on ⅓ of dental plate.

V
. Vertebrae zygospondylous; beyond segment 5 with short keel; without oral bridge.

D
. Disc pentagonal or circular; high (above arm level). Integument obscuring plates. Dorsal disc with granules; with concealed primary plates; with scales; without spines. Dorsal disc scale / plate without tubercles. Radial shield present; with granules; length more than half of the disc radius; scalene (oblique) triangular.

V
. Gonads restricted to the disc. Ventral interradii with spines. Scales on ventral interradii absent. Ventral teeth several (tooth papillae). Infradental papillae on oral plate present. Buccal scales on OP absent. Accessory oral papillae on OP absent. Lateral oral papillae on OP present; single row of lateral papillae; spiniform. Adoral shield spine present. Lyman's ossicle present. Oral shield covering more than one third of the length of an interradius; about as long as wide. Madreporite no data. Adoral shield separated. Abradial genital plate not twisted. Number of genital slits one per bursa. Genital slit longer than half the length of an interradius. Genital papillae absent. Dental plate dorsal half widest; entire. Dental sockets heart-shaped(?); equal to or more than 50% of the width. Teeth fl at (with round or slightly pointed tip, but never spine-like).

V
. Vertebrae zygospondylous; beyond segment 5 with short keel; without oral bridge. Arm spine cross section oval. The ratio of thickness of distal ⅓ of arm spine to maximum thickness (in a given cross-sectional plane) 0.6-0.9. Arm spines fl at; mainly lateral; predominantly erect, standing perpendicular to arm axis. Longest arm spines longer than two segments. Tentacle scales present; pores visible along most of the arm; more than two times as long as wide; shorter than the spine nearby; cross section oval. Position of lateral arm plate (LAP) arched (wrapped around the arm). Dorsal and / or ventral edge(s) of LAPs with constriction. Ventral position of LAP projecting ventro-proximalwards. Ventrodistal tip of LAP not projecting ventralwards. Arm spine articulation with dorsal and ventral lobes; with a fully developed sigmoidal fold; arranged over entire distal LAP edge; dorsalwards increasing in size. Distance between spine articulations equal. Nerve and muscle openings separated by large, prominent ridge or regular stereom. Nerve opening smaller than muscle opening. D . Disc circular; fl at (about same level as arms). Integument obscuring plates. Dorsal disc with granules; with concealed primary plates; with scales; without spines. Dorsal disc scale / plate without tubercles. Radial shield present; with granules; length more than half of the disc radius.

V
. Gonads restricted to the disc. Ventral interradii with granules. Scales on ventral interradii covering the whole area of an interradius. The number of scales on H line of an interradius no data. Ventral teeth several (tooth papillae). Infradental papillae on oral plate present. Buccal scales on OP absent. Accessory oral papillae on OP absent. Lateral oral papillae on OP present; single row of lateral papillae; paddle-shaped. Adoral shield spine present. Lyman's ossicle present. Oral shield covering less than one third of interradius; about as long as wide. Madreporite no data. Adoral shield meeting in front of the oral shield. Abradial genital plate not twisted. Number of genital slits one per bursa. Genital slit longer than half the length of an interradius. Genital papillae absent. Dental plate of equal width all over; entire (single plate). Dental sockets slit-shaped and circular; equal to or more than 50% of the width. At least one socket on DP as perforation with septum. Teeth fl at (with round or slightly pointed tip, but never spine-like). Teeth on DP arranged as a single row on ⅔ and a cluster of tooth papillae on ⅓ of dental plate.

V
. Vertebrae zygospondylous; beyond segment 5 with short keel; without oral bridge.

Description
A . Arm shape unbranched; length 3 × disc diameter, or length 3-4 × disc diameter, or length more than 4 × disc diameter; covering without granules / spines / tubercles. Arm combs absent. Dorsal arm plate present. Accessory dorsal arm plate absent. Ventral arm plate present. Accessory ventral arm plate absent. Girdle of tiny hooks on the arms absent. Spines all similar in shape. Arm spine cross section round. The ratio of thickness of distal ⅓ of arm spine to maximum thickness (in a given cross-sectional plane) 0.6-0.9, or 0.9-1 (Ophiochondrus). Arm spines pointed; mainly lateral; predominantly erect, standing perpendicular to arm axis. Longest arm spines between one and two segments long (Ophiolimna), or longer than two segments. Tentacle scales present; pores visible along most of the arm; nearly as long as wide; shorter than the spine nearby; cross section oval. Buccal scales on OP absent. Accessory oral papillae on OP absent. Lateral oral papillae on OP present; single row of lateral papillae; spiniform. Adoral shield spine present. Lyman's ossicle present. Oral shield covering less than one third of interradius; much wider than long. Madreporite one or multiple; with one pore or with numerous pores. Adoral shield meeting in front of the oral shield. Abradial genital plate not twisted. Number of genital slits one per bursa. Genital slit longer than half the length of an interradius. Genital papillae absent. Dental plate of equal width all over, or dorsal half widest (Ophiolimna); entire (single plate). Dental sockets slit-shaped; equal to or more than 50% of the width. At least one socket on DP as depression or perforation without septum. Teeth fl at (with round or slightly pointed tip, but never spine-like). Teeth on DP arranged as single row.

V
. Vertebrae zygospondylous; beyond segment 5 with short keel; without oral bridge.

Description
A . Arm shape unbranched; length 3-4 × disc diameter; covering without granules / spines / tubercles. Arm combs absent. Dorsal arm plate present. Accessory dorsal arm plate absent. Ventral arm plate present. Accessory ventral arm plate absent. Girdle of tiny hooks on the arms absent. Spines all similar in shape. Arm spine cross section oval. The ratio of thickness of distal ⅓ of arm spine to maximum thickness (in a given cross-sectional plane) 0.6-0.9. Arm spines fl at; mainly lateral; predominantly parallel to arm axis (adpressed). Longest arm spines shorter than half a segment. Tentacle scales present; pores visible along most of the arm; slightly longer than wide; shorter than the spine nearby; cross section oval. Position of lateral arm plate (LAP) arched (wrapped around the arm). Dorsal and / or ventral edge(s) of LAPs without constriction. Ventral position of LAP projecting ventro-proximalwards. Ventro-distal tip of LAP projecting ventralwards. Arm spine articulation with dorsal and ventral lobes; with a weakly developed sigmoidal fold; arranged over entire distal LAP edge; ventralwards increasing in size. Distance between spine articulations equal. Nerve and muscle openings separated by small ridge if at all. Nerve opening smaller than muscle opening.

D
. Disc pentagonal to circular; fl at (about same level as arms). Integument obscuring plates. Dorsal disc with granules; with concealed primary plates; with scales; without spines. Dorsal disc scale / plate without tubercles. Radial shield present; with granules; length less than one third of the disc radius; isosceles (mirror-symmetric) triangular to pear-shaped.

V
. Gonads restricted to the disc. Ventral interradii with granules. Scales on ventral interradii absent. Ventral teeth several (tooth papillae). Infradental papillae on oral plate present. Buccal scales on OP absent. Accessory oral papillae on OP absent. Lateral oral papillae on OP present; single row of lateral papillae; spiniform. Adoral shield spine present. Lyman's ossicle present. Oral shield covering less than one third of interradius; much wider than long. Madreporite multiple; with one pore. Adoral shield meeting in front of the oral shield. Abradial genital plate not twisted. Number of genital slits two per bursa. Genital slit longer than half the length of an interradius. Genital papillae absent. Dental plate of equal width all over; fragmented. Dental sockets slit-shaped and circular; less than 50% of the width or equal to or more than 50% of the width. At least one socket on DP as depression or perforation without septum. Teeth block-like (with square tip). Teeth on DP arranged as a single row on ⅔ and a cluster of tooth papillae on ⅓ of dental plate.

V
. Vertebrae zygospondylous; beyond segment 5 with short keel; without oral bridge.

Description
A . Arm shape unbranched; length more than 4 × disc diameter; covering without granules / spines / tubercles. Arm combs absent. Dorsal arm plate present. Accessory dorsal arm plate absent. Ventral arm plate present. Accessory ventral arm plate absent. Girdle of tiny hooks on the arms absent. Spines all similar in shape. Arm spine cross section round. The ratio of thickness of distal ⅓ of arm spine to maximum thickness (in a given cross-sectional plane) 0.6-0.9. Arm spines pointed; mainly lateral; predominantly parallel to arm axis (adpressed). Longest arm spines shorter than half a segment. Tentacle scales present; pores visible along most of the arm; slightly longer than wide; shorter than the spine nearby. Dental plate ventral half widest; entire (single plate). Position of lateral arm plate (LAP) only lateral. Dorsal and / or ventral edge(s) of LAPs without constriction. Ventral position of LAP not projecting ventro-proximalwards. Ventro-distal tip of LAP not projecting ventralwards. Arm spine articulation with dorsal and thin ventral lobes and a fully developed sigmoidal fold; arranged over entire distal LAP edge; larger in middle of arm. Distance between spine articulations equal. Nerve and muscle openings separated by large, prominent ridge or regular stereom. Nerve opening smaller than muscle opening. D . Disc pentagonal; fl at (about same level as arms). Integument obscuring plates. Dorsal disc with granules; with concealed primary plates; without scales; with spines. Dorsal disc scale / plate without tubercles. Radial shield present; covering, length and shape no data.

V
. Gonads restricted to the disc. Ventral interradii with granules. Scales on ventral interradii absent. Ventral teeth several (tooth papillae). Infradental papillae on oral plate present. Buccal scales on OP absent. Accessory oral papillae on OP absent. Lateral oral papillae on OP present; single row of lateral papillae; spiniform. Adoral shield spine present. Lyman's ossicle present. Oral shield covering less than one third of interradius; about as long as wide. Madreporite no data. Adoral shield separated. Abradial genital plate not twisted. Number of genital slits one per bursa. Genital slit longer than half the length of an interradius. Genital papillae absent. Dental sockets slit-shaped or circular; less than 50% of the width. At least one socket on DP as depression or perforation without septum. Teeth spine-shaped. Teeth on DP arranged as a single row on ⅔ and a cluster of tooth papillae on ⅓ of dental plate.

Description
A . Arm shape unbranched; length 3-4 × disc diameter; covering without granules / spines / tubercles. Arm combs absent. Dorsal arm plate present. Accessory dorsal arm plate absent. Ventral arm plate present. Accessory ventral arm plate absent. Girdle of tiny hooks on the arms absent. Spines all similar in shape. Arm spine cross section round. The ratio of thickness of distal ⅓ of arm spine to maximum thickness (in a given cross-sectional plane) 0.6-0.9. Arm spines pointed; mainly lateral; predominantly erect, standing perpendicular to arm axis. Longest arm spines shorter than half a segment, or longer than two segments (Ophiarachna). Tentacle scales present; pores visible along most of the arm; slightly longer than wide, or more than two times as long as wide; shorter than the spine nearby; cross section oval. Position of lateral arm plate (LAP) arched (wrapped around the arm). Dorsal and / or ventral edge(s) of LAPs without constriction. Ventral position of LAP projecting ventro-proximalwards. Ventro-distal tip of LAP projecting ventralwards. Arm spine articulation with dorsal and ventral lobes; without or with a weakly developed sigmoidal fold; arranged over entire distal LAP edge; ventralwards increasing in size. Distance between spine articulations equal. Nerve and muscle openings separated by small ridge if at all. Nerve opening approximately as large as muscle opening. D . Disc pentagonal; fl at (about same level as arms). Integument not obscuring plates. Dorsal disc with granules (Ophiarachna) or without granules; with concealed primary plates; with scales; without spines. Dorsal disc scale / plate without tubercles. Radial shield present; without granules / spines; length less than one third of the disc radius; isosceles (mirror-symmetric) triangular to pear-shaped.

V
. Gonads restricted to the disc. Ventral interradii without granules / spines, or with granules (Ophiarachna). Scales on ventral interradii absent. Ventral teeth single or with several tooth papillae (Ophiarachna). Infradental papillae on oral plate present. Buccal scales on OP absent. Accessory oral papillae on OP absent. Lateral oral papillae on OP present; single row of lateral papillae; paddle-shaped. Adoral shield spine present. Lyman's ossicle present. Oral shield covering less than one third of interradius; about as long as wide. Madreporite one; with one pore. Adoral shield separated. Abradial genital plate not twisted. Number of genital slits one per bursa. Genital slit longer than half the length of an interradius. Genital papillae absent. Dental plate ventral half widest; fragmented. Dental sockets slit-shaped and circular; less than 50% of the width or equal to or more than 50% of the width. At least one socket on DP as perforation with septum. Teeth block-like (with square tip). Teeth on DP arranged as single row.

V
. Vertebrae zygospondylous; beyond segment 5 with short keel; without oral bridge.

Description
A . Arm shape unbranched; length 3-4 × disc diameter; covering without granules / spines / tubercles. Arm combs absent. Dorsal arm plate present. Accessory dorsal arm plate absent. Ventral arm plate present. Accessory ventral arm plate absent. Girdle of tiny hooks on the arms absent. Spines all similar in shape. Arm spine cross section oval. The ratio of thickness of distal ⅓ of arm spine to maximum thickness (in a given cross-sectional plane) 0.6-0.9, or 0.9-1. Arm spines fl at, or cylindrical, or pointed; mainly lateral; predominantly erect, standing perpendicular to arm axis. Longest arm spines longer than two segments. Tentacle scales present; pores visible along most of the arm; nearly as long as wide; shorter than the spine nearby; cross section oval. Position of lateral arm plate (LAP) arched (wrapped around the arm). Dorsal and / or ventral edge(s) of LAPs without constriction. Ventral position of LAP projecting ventro-proximalwards. Ventro-distal tip of LAP not projecting ventralwards. Arm spine articulation with dorsal and ventral lobes; with a fully developed sigmoidal fold; arranged over entire distal LAP edge; dorsalwards increasing in size. Distance between spine articulations dorsalwards increasing. Nerve and muscle openings separated by small ridge if at all. Nerve opening smaller than muscle opening.

D
. Disc pentagonal to circular; fl at (about same level as arms). Integument obscuring plates. Dorsal disc with granules; with concealed primary plates; with scales; with smooth spines (O'Hara et al. 2018) or without spines. Dorsal disc scale / plate without tubercles. Radial shield present; with granules; length less than one third of the disc radius; scalene (oblique) triangular.

V
. Gonads restricted to the disc. Ventral interradii with granules. Scales on ventral interradii covering the whole area of an interradius. The number of scales on H line of an interradius more than 21. Ventral teeth several (tooth papillae). Infradental papillae on oral plate present. Buccal scales on OP absent. Accessory oral papillae on OP absent. Lateral oral papillae on OP present or absent; multiple rows of lateral papillae; rounded. Adoral shield spine present. Lyman's ossicle present. Oral shield covering less than one third of interradius; about as long as wide. Madreporite one; with one pore. Adoral shield separated. Abradial genital plate not twisted. Number of genital slits one per bursa. Genital slit longer than half the length of an interradius. Genital papillae absent. Dental plate of equal width all over; entire (single plate). Dental sockets circular to heart-shaped; equal to or more than 50% of the width. At least one socket on DP as perforation with septum. Teeth block-like (with square tip). Teeth on DP arranged as a single row on half of dental plate and other half with a cluster of tooth papillae (ventral cluster), or a single row on ⅔ and a cluster of tooth papillae on ⅓ of dental plate.

V
. Vertebrae zygospondylous; beyond segment 5 with short keel; without oral bridge.  (2021) 38 Accessory ventral arm plate absent. Girdle of tiny hooks on the arms absent. Spines not all similar in shape. The atypical spines are bristles (Fig. 2H). Arm spine cross section oval. The ratio of thickness of distal ⅓ of arm spine to maximum thickness (in a given cross-sectional plane) 0.6-0.9. Arm spines pointed; mainly lateral; predominantly parallel to arm axis (adpressed). Longest arm spines between half and one segment long. Tentacle scales present; pores visible along most of the arm; slightly longer than wide; shorter than the spine nearby; cross section oval. Position of lateral arm plate (LAP) only lateral. Dorsal and / or ventral edge(s) of LAPs with constriction. Ventral position of LAP not projecting ventro-proximalwards. Ventro-distal tip of LAP projecting ventralwards. Arm spine articulation without dorsal and ventral lobes or with weak lobes (O'Hara et al. 2018); arranged over entire distal LAP edge; dorsalwards increasing in size. Distance between spine articulations dorsalwards increasing. Nerve and muscle openings separated by large, prominent ridge or regular stereom. Nerve opening approximately as large as muscle opening.
D . Disc pentagonal to circular; fl at (about same level as arms). Integument obscuring plates. Dorsal disc with granules; with concealed primary plates; with scales; without spines. Dorsal disc scale / plate without tubercles. Radial shield present; without granules / spines; length less than one third of the disc radius; isosceles (mirror-symmetric) triangular to pear-shaped.

V
. Gonads restricted to the disc. Ventral interradii with granules. Scales on ventral interradii covering the whole area of an interradius. The number of scales on H line of an interradius fewer than 7. Ventral teeth several (tooth papillae). Infradental papillae on oral plate present. Buccal scales on OP absent. Accessory oral papillae on OP absent. Lateral oral papillae on OP present; single row of lateral papillae; spiniform. Adoral shield spine present. Lyman's ossicle present. Oral shield covering less than one third of interradius; about as long as wide. Madreporite one; with one pore. Adoral shield meeting in front of the oral shield. Abradial genital plate not twisted. Number of genital slits one per bursa. Genital slit longer than half the length of an interradius. Genital papillae absent. Dental plate of equal width all over; entire (single plate). Dental sockets circular; equal to or more than 50% of the width. At least one socket on DP as depression or perforation without septum. Teeth fl at (with round or slightly pointed tip, but never spine-like). Teeth on DP arranged as single row.

V
. Vertebrae zygospondylous; beyond segment 5 with short keel; without oral bridge.

Description
A . Arm shape unbranched; length more than 4 × disc diameter; covering without granules / spines / tubercles. Arm combs absent. Dorsal arm plate present. Accessory dorsal arm plate absent. Ventral arm plate present. Accessory ventral arm plate absent. Girdle of tiny hooks on the arms absent. Spines all similar or not all similar in shape (Ophiopallas). The atypical spines are hook-shaped. Arm spine cross section round. The ratio of thickness of distal ⅓ of arm spine to maximum thickness (in a given cross-sectional plane) 0.6-0.9. Arm spines pointed; mainly lateral; predominantly parallel to arm axis (adpressed). Longest arm spines between half and one segment long. Tentacle scales present; pores visible along most of the arm; nearly as long as wide; shorter than the spine nearby; cross section round. Position of lateral arm plate (LAP) arched (wrapped around the arm). Dorsal and / or ventral edge(s) of LAPs without constriction. Ventral position of LAP projecting ventro-proximalwards. Ventro-distal tip of LAP projecting ventralwards. Arm spine articulation with dorsal and ventral lobes; without sigmoidal fold; arranged over entire distal LAP edge; larger in middle of arm. Distance between spine articulations dorsalwards increasing, or not applicable (two spine articulations) (Ophioleuce seminudum Koehler, 1904). Nerve and muscle openings separated by small ridge if at all. Nerve opening smaller than muscle opening. D . Disc circular; fl at (about same level as arms). Integument obscuring plates. Dorsal disc with granules; with concealed primary plates; with scales; without spines. Dorsal disc scale / plate without tubercles. Radial shield present; with granules; length less than one third of the disc radius; scalene (oblique) triangular.

V
. Gonads restricted to the disc. Ventral interradii with granules. Scales on ventral interradii absent. Ventral teeth several (tooth papillae). Infradental papillae on oral plate present. Buccal scales on OP absent. Accessory oral papillae on OP absent. Lateral oral papillae on OP absent. Adoral shield spine absent. Lyman's ossicle present. Oral shield covering less than one third of interradius; much longer than wide. Madreporite multiple; with numerous pores. Adoral shield meeting in front of the oral shield. Abradial genital plate not twisted. Number of genital slits one per bursa. Genital slit longer than half the length of an interradius. Genital papillae present (Ophiopallas) or absent. Dental plate ventral half widest; entire (single plate). Dental sockets slit-shaped; less than 50% of the width. At least one socket on DP as depression or perforation without septum. Teeth spine-shaped. Teeth on DP arranged as single row.

Description
A . Arm shape unbranched; length 3 × disc diameter; covering without granules / spines / tubercles. Arm combs absent. Dorsal arm plate present. Accessory dorsal arm plate present. Ventral arm plate present. Accessory ventral arm plate absent or present in other genera or species of Ophiolepis (Koehler 1922 andPineda-Enriquez 2014). Girdle of tiny hooks on the arms absent. Spines all similar in shape. Arm spine cross section round. The ratio of thickness of distal ⅓ of arm spine to maximum thickness (in a given cross-sectional plane) 0.6-0.9. Arm spines pointed; mainly lateral; predominantly parallel to arm axis (adpressed). Longest arm spines shorter than half a segment. Tentacle scales present; pores visible along most of the arm; slightly longer than wide; shorter than the spine nearby; cross section oval. Position of lateral arm plate (LAP) arched (wrapped around the arm). Dorsal and / or ventral edge(s) of LAPs without constriction. Ventral position of LAP projecting ventro-proximalwards. Ventro-distal tip of LAP not projecting ventralwards. Arm spine articulation with dorsal and ventral lobes; without sigmoidal fold; arranged over entire distal LAP edge; larger in middle of arm. Distance between spine articulations dorsalwards increasing. Nerve and muscle openings separated by small ridge if at all. Nerve opening approximately as large as muscle opening.
D . Disc pentagonal to circular; fl at (about same level as arms). Integument not obscuring plates. Dorsal disc without granules; with obvious primary plates; with scales; without spines. Dorsal disc scale / plate without tubercles. Radial shield present; without granules / spines; length between one third and half of the disc radius; isosceles (mirror-symmetric) triangular to pear-shaped.

V
. Gonads restricted to the disc. Ventral interradii without granules / spines. Scales on ventral interradii absent. Ventral teeth single. Infradental papillae on oral plate present. Buccal scales on OP present. Accessory oral papillae on OP absent. Lateral oral papillae on OP absent. Adoral shield spine present. Lyman's ossicle present. Oral shield covering less than one third of interradius; much longer than wide. Madreporite one. Adoral shield meeting in front of the oral shield. Abradial genital plate not twisted. Number of genital slits one per bursa. Genital slit longer than half the length of an interradius. Genital papillae absent. Dental plate of equal width all over, or dorsal half widest; entire (single plate). Dental sockets circular to heart-shaped; equal to or more than 50% of the width. At least one socket on DP as depression or perforation without septum. Teeth block-like (with square tip). Teeth on DP arranged as single row.

V
. Vertebrae zygospondylous; beyond segment 5 with short keel; without oral bridge.

Description
A . Arm shape unbranched; length more than 4 × disc diameter; covering without granules / spines / tubercles. Arm combs absent. Dorsal arm plate present. Accessory dorsal arm plate present or absent (Actinozonella). Ventral arm plate present. Accessory ventral arm plate absent. Girdle of tiny hooks on the arms absent. Spines all similar in shape. Arm spine cross section round or oval (Sigbeia). The ratio of thickness of distal ⅓ of arm spine to maximum thickness (in a given cross-sectional plane) 0.9-1. Arm spines cylindrical; at proximal segments only on ventral side of arms; predominantly parallel to arm axis (adpressed). Longest arm spines shorter than half a segment (Sigsbeia, Ophiozonella falklandica Mortensen, 1936), or between half and one segment long (Hemieuryale, Ophiozonella longispina (H.L. Clark, 1908), Ophioplus). Tentacle scales present; pores visible along most of the arm; nearly as long as wide; shorter than the spine nearby; cross section oval. Position of lateral arm plate (LAP) only lateral or arched (wrapped around the arm) (Sigsbeia). Dorsal and / or ventral edge(s) of LAPs without constriction. Ventral position of LAP projecting ventro-proximalwards. Ventro-distal tip of LAP not projecting ventralwards. Arm spine articulation with dorsal and ventral lobes; without sigmoidal fold; restricted to (the ventral or central) portion of the distal edge; ventralwards increasing in size. Distance between spine articulations not applicable (two spine articulations). Nerve and muscle openings separated by small ridge if at all. Nerve opening smaller than muscle opening, or approximately as large as muscle opening (Ophiozonella).

D
. Disc pentagonal to circular; high (above arm level). Integument not obscuring plates. Dorsal disc without granules; with obvious primary plates; with scales; without spines. Dorsal disc scale / plate with tubercles. Radial shield present; without granules / spines; length between one third and half of the disc radius, or length more than half of the disc radius (Ophioplus); isosceles (mirror-symmetric) triangular to pear-shaped.

V
. Gonads restricted to the disc. Ventral interradii without granules / spines. Scales on ventral interradii covering the whole area of an interradius. The number of scales on H line of an interradius fewer than 7. Ventral teeth single. Infradental papillae on oral plate present. Buccal scales on OP absent. Accessory oral papillae on OP absent. Lateral oral papillae on OP present; one lateral papilla along each side of the jaw; block-shaped. Adoral shield spine present. Lyman's ossicle present. Oral shield covering less than one third of interradius; much wider than long. Madreporite no data. Adoral shield meeting in front of the oral shield. Abradial genital plate not twisted. Number of genital slits one per bursa. Genital slit shorter than half interradius. Genital papillae absent. Dental plate dorsal half widest; entire (single plate). Dental sockets slit-shaped and circular; equal to or more than 50% of the width. At least one socket on DP as depression or perforation without septum. Teeth fl at (with round or slightly pointed tip, but never spine-like). Teeth on DP arranged as single row.

Description
A . Arm shape unbranched; length more than 4 × disc diameter; covering without granules / spines / tubercles. Arm combs absent. Dorsal arm plate present. Accessory dorsal arm plate absent. Ventral arm plate present. Accessory ventral arm plate absent. Girdle of tiny hooks on the arms absent. Spines all similar in shape. Arm spine cross section oval. The ratio of thickness of distal ⅓ of arm spine to maximum thickness (in a given cross-sectional plane) 0.6-0.9. Arm spines fl at; mainly lateral; predominantly erect, standing perpendicular to arm axis. Longest arm spines between one and two segments long. Tentacle scales present; pores visible along most of the arm; more than two times as long as wide; shorter than the spine nearby; cross section round. Position of lateral arm plate (LAP) arched (wrapped around the arm). Dorsal and / or ventral edge(s) of LAPs without constriction. Ventral position of LAP projecting ventroproximalwards. Ventro-distal tip of LAP projecting ventralwards. Arm spine articulation with dorsal and ventral lobes; without sigmoidal fold; arranged over entire distal LAP edge; ventralwards increasing in size. Distance between spine articulations dorsalwards increasing. Nerve and muscle openings separated by small ridge if at all. Nerve opening approximately as large as muscle opening.

D
. Disc pentagonal or circular; fl at (about same level as arms). Integument obscuring plates. Dorsal disc without granules; with concealed primary plates; with scales; with spines. Dorsal disc scale / plate without tubercles. Radial shield present; without granules / spines; length between one third and half of the disc radius; scalene (oblique) triangular or bar-like.

V
. Gonads restricted to the disc. Ventral interradii with spines. Scales on ventral interradii covering the whole area of an interradius. The number of scales on H line of an interradius about 14-21.
Ventral teeth single. Infradental papillae on oral plate present. Buccal scales on OP present. Accessory oral papillae on OP absent. Lateral oral papillae on OP present; single row of lateral papillae; rounded and spiniform. Adoral shield spine present. Lyman's ossicle present. Oral shield covering less than one third of interradius; about as long as wide. Madreporite no data. Adoral shield meeting in front of the oral shield. Abradial genital plate twisted. Number of genital slits one per bursa. Genital slit longer than half the length of an interradius. Genital papillae absent. Dental plate dorsal half widest; entire (single plate). Dental sockets slit-shaped and circular; equal to or more than 50% of the width. At least one socket on DP as depression or perforation without septum. Teeth fl at (with round or slightly pointed tip, but never spine-like). Teeth on DP arranged as single row.

V
. Vertebrae zygospondylous; beyond segment 5 with short keel; without oral bridge.

Description
A . Arm shape unbranched; length more than 4 × disc diameter; covering without granules / spines / tubercles. Arm combs absent. Dorsal arm plate present. Accessory dorsal arm plate present or sometimes absent (Ophiodoris sp.). Ventral arm plate present. Accessory ventral arm plate absent. Girdle of tiny hooks on the arms absent. Spines all similar in shape. Arm spine cross section round. The ratio of thickness of distal ⅓ of arm spine to maximum thickness (in a given cross-sectional plane) 0.6-0.9, or 0.9-1 (Ophioplax, Ophiodoris). Arm spines pointed or cylindrical (Ophioplax, Ophiodoris); mainly lateral; predominantly erect, standing perpendicular to arm axis. Longest arm spines between half and one segment long, or between one and two segments long (Ophioplax). Tentacle scales present; pores visible along most of the arm; slightly longer than wide; shorter than the spine nearby; cross section oval or round. Position of lateral arm plate (LAP) arched (wrapped around the arm). Dorsal and / or ventral edge(s) of LAPs without constriction. Ventral position of LAP projecting ventro-proximalwards. Ventro-distal tip of LAP not projecting ventralwards. Arm spine articulation with dorsal and ventral lobes; without sigmoidal fold; arranged over entire distal LAP edge; larger in middle of arm. Distance between spine articulations dorsalwards increasing. Nerve and muscle openings separated by small ridge if at all. Nerve opening smaller than muscle opening.

D
. Disc pentagonal to circular; fl at (about same level as arms) to high (above arm level). Integument obscuring plates. Dorsal disc without granules or with granules along the genital slit, extending onto disc edge; with concealed primary plates; with scales; with spines (Ophiodoris) or without spines. Dorsal disc scale / plate without tubercles. Radial shield present; without granules / spines; length less than one third of the disc radius; isosceles (mirror-symmetric) triangular to pear-shaped.

V
. Gonads restricted to the disc. Ventral interradii without granules / spines or with granules (Ophioplax). Scales on ventral interradii covering the whole area of an interradius or absent (Ophioplax, Ophiodoris). The number of scales on H line of an interradius about 14-21. Ventral teeth single. Infradental papillae on oral plate present. Buccal scales on OP present. Accessory oral papillae on OP absent. Lateral oral papillae on OP absent. Adoral shield spine present. Lyman's ossicle present. Oral shield covering less than one third of interradius; about as long as wide. Madreporite one; with one pore. Adoral shield separated. Abradial genital plate not twisted. Number of genital slits one per bursa. Genital slit longer than half the length of an interradius. Genital papillae present. Dental plate of equal width all over, or ventral half widest (Ophioplax); entire (single plate). Dental sockets circular to heart-shaped; equal to or more than 50% of the width. At least one socket on DP as perforation with septum. Teeth block-like (with square tip). Teeth on DP arranged as single row.

V
. Vertebrae zygospondylous; beyond segment 5 with extended keel, or with short keel; without oral bridge.

Description
A . Arm shape unbranched; length more than 4 × disc diameter; covering without granules / spines / tubercles. Arm combs absent. Dorsal arm plate present. Accessory dorsal arm plate absent. Ventral arm plate present. Accessory ventral arm plate absent. Girdle of tiny hooks on the arms absent. Spines all similar in shape. Arm spine cross section oval. The ratio of thickness of distal ⅓ of arm spine to maximum thickness (in a given cross-sectional plane) 0.9-1. Arm spines fl at; mainly lateral; predominantly erect, standing perpendicular to arm axis. Longest arm spines between one and two segments long. Tentacle scales present; pores visible along most of the arm; more than two times as long as wide; longer than the spine nearby; cross section oval. Position of lateral arm plate (LAP) arched (wrapped around the arm). Dorsal and / or ventral edge(s) of LAPs without constriction. Ventral position of LAP projecting ventroproximalwards. Ventro-distal tip of LAP projecting ventralwards. Arm spine articulation with dorsal and ventral lobes; without sigmoidal fold; arranged over entire distal LAP edge; ventralwards increasing in size. Distance between spine articulations dorsalwards increasing. Nerve and muscle openings separated by small ridge if at all. Nerve opening approximately as large as muscle opening.

D
. Disc circular; fl at (about same level as arms). Integument obscuring plates. Dorsal disc without granules; with concealed primary plates; with scales; without spines. Dorsal disc scale / plate without tubercles. Radial shield present; without granules / spines; length between one third and half of the disc radius; bar-like.

V
. Gonads restricted to the disc. Ventral interradii without granules / spines. Scales on ventral interradii covering the whole area of an interradius. The number of scales on H line of an interradius more than 21. Ventral teeth several (tooth papillae). Infradental papillae on oral plate present. Buccal scales on OP present. Accessory oral papillae on OP absent. Lateral oral papillae on OP absent. Adoral shield spine present. Lyman's ossicle present. Oral shield covering more than one third of the length of an interradius; about as long as wide. Madreporite multiple; with numerous pores. Adoral shield meeting in front of the oral shield. Abradial genital plate not twisted. Number of genital slits one per bursa. Genital slit longer than half the length of an interradius. Genital papillae absent. Dental plate dorsal half widest; entire (single plate). Dental sockets slit-shaped; equal to or more than 50% of the width. At least one socket on DP as perforation with septum. Teeth block-like (with square tip). Teeth on DP arranged as a single row on ⅔ and a cluster of tooth papillae on ⅓ of dental plate.

V
. Vertebrae zygospondylous; beyond segment 5 with short keel; without oral bridge.

Description
A . Arm shape unbranched; length 3-4 × or more than 4 × disc diameter; covering without granules / spines / tubercles. Arm combs absent. Dorsal arm plate present. Accessory dorsal arm plate absent. Ventral arm plate present. Accessory ventral arm plate absent. Girdle of tiny hooks on the arms absent. Spines all similar in shape. Arm spine cross section round. The ratio of thickness of distal ⅓ of arm spine to maximum thickness (in a given cross-sectional plane) 0.6-0.9, or 0.9-1. Arm spines tapering (conical), cylindrical, or pointed; mainly lateral; predominantly parallel to arm axis (adpressed). Longest arm spines between half and one, or between one and two segments long. Tentacle scales present or absent (Ophiophragmus); pores only proximally visible along the arm; nearly as long as wide; shorter than the spine nearby; cross section oval. Position of lateral arm plate (LAP) only lateral. Dorsal and / or ventral edge(s) of LAPs without constriction. Ventral position of LAP not projecting ventro-proximalwards. Ventro-distal tip of LAP not projecting ventralwards. Arm spine articulation without dorsal and ventral lobes, or with dorsal and ventral lobes; without sigmoidal fold; arranged over entire distal LAP edge; ventralwards increasing in size. Distance between spine articulations dorsalwards increasing.. Nerve and muscle openings separated by small ridge if at all. Nerve opening approximately as large as muscle opening.
D . Disc pentagonal to circular; fl at (about same level as arms) to high (above arm level). Integument not obscuring plates. Dorsal disc without granules; with obvious primary plates or with concealed primary plates; without scales; without spines or with spines, granules and tubercles in some genera (O'Hara et al. 2018). Dorsal disc scale / plate without tubercles. Radial shield present; without granules / spines; length between one third and half of the disc radius; scalene (oblique) triangular, or isosceles (mirror-symmetric) triangular to pear-shaped, or bar-like.

V
. Gonads restricted to the disc. Ventral interradii without granules / spines or with spines. Scales on ventral interradii covering the whole area of interradius or absent. The number of scales on H line of an interradius about 14-21. Ventral teeth single. Infradental papillae on oral plate present. Buccal scales on OP present. Accessory oral papillae on OP absent. Lateral oral papillae on OP absent. Adoral shield spine present. Lyman's ossicle present. Oral shield covering less than one third of interradius; much wider than long or about as long as wide (Amphipholis). Madreporite one; with one pore or with numerous pores. Adoral shield separated or meeting in front of the oral shield. Abradial genital plate not twisted. Number of genital slits one per bursa. Genital slit longer than half the length of an interradius. Genital papillae absent. Dental plate dorsal half widest, entire (single plate). Dental sockets slit-shaped to heart-shaped, equal to or more that 50% of the width of dental plate. At least one socket on DP as perforation with septum. Teeth block-like (with square tip). Teeth on DP arranged as single column.

V
. Vertebrae zygospondylous; beyond segment 5 with short keel; without oral bridge.

Description
A . Arm shape unbranched; length more than 4 × disc diameter; covering without granules / spines / tubercles. Arm combs absent. Dorsal arm plate present. Accessory dorsal arm plate absent. Ventral arm plate present. Accessory ventral arm plate absent. Girdle of tiny hooks on the arms absent. Spines all similar in shape. Arm spine cross section round. The ratio of thickness of distal ⅓ of arm spine to maximum thickness (in a given cross-sectional plane) 0.6-0.9. Arm spines pointed; mainly lateral; predominantly erect, standing perpendicular to arm axis. Longest arm spines between one and two segments long. Tentacle scales absent. Position of lateral arm plate (LAP) arched (wrapped around the arm). Dorsal and / or ventral edge(s) of LAPs without constriction. Ventral position of LAP projecting ventro-proximalwards. Ventro-distal tip of LAP not projecting ventralwards. Arm spine articulation with dorsal and ventral lobes; without sigmoidal fold; arranged over entire distal LAP edge; ventralwards increasing in size. Distance between spine articulations dorsalwards increasing. Nerve and muscle openings separated by small ridge if at all. Nerve opening smaller than muscle opening. D . Disc pentagonal to circular; fl at (about same level as arms). Integument not obscuring plates. Dorsal disc without granules; with obvious primary plates; with scales; without spines. Dorsal disc scale / plate without tubercles. Radial shield present; without granules / spines; length more than half of the disc radius; scalene (oblique) triangular.

V
. Gonads restricted to the disc. Ventral interradii without granules / spines. Scales on ventral interradii covering the whole area of an interradius. The number of scales on H line of an interradius about 14-21. Ventral teeth single. Infradental papillae on oral plate present. Buccal scales on OP present. Accessory oral papillae on OP absent. Lateral oral papillae on OP absent. Adoral shield spine present. Lyman's ossicle present. Oral shield covering less than one third of interradius; much wider than long. Madreporite one; with one pore. Adoral shield meeting in front of the oral shield. Abradial genital plate not twisted. Number of genital slits one per bursa. Genital slit longer than half the length of an interradius. Genital papillae absent. Dental plate of equal width all over; entire (single plate). Dental sockets slit-shaped and heart-shaped; less than 50% of the width. At least one socket on DP as depression or perforation without septum. Teeth fl at (with round or slightly pointed tip, but never spine-like). Teeth on DP arranged as single row.

V
. Vertebrae zygospondylous; beyond segment 5 with short keel; without oral bridge. D . Disc pentagonal to circular; fl at (about same level as arms). Integument not obscuring plates. Dorsal disc without granules; with concealed primary plates; with scales; with spines or without spines. Dorsal disc scale / plate without tubercles. Radial shield present; without granules / spines; length between one third and half of the disc radius; scalene (oblique) triangular.

V
. Gonads restricted to the disc. Ventral interradii without granules / spines. Scales on ventral interradii covering the whole area of an interradius. The number of scales on H line of an interradius about 7-14. Ventral teeth single. Infradental papillae on oral plate present. Buccal scales on OP present. Accessory oral papillae on OP absent. Lateral oral papillae on OP absent. Adoral shield spine present. Lyman's ossicle present. Oral shield covering less than one third of interradius; about as long as wide. Madreporite one; with one pore. Adoral shield meeting in front of the oral shield. Abradial genital plate not twisted. Number of genital slits one per bursa. Genital slit longer than half the length of an interradius. Genital papillae absent. Dental plate ventral half widest; entire (single plate). Dental sockets circular; less than 50% of the width. At least one socket on DP as depression or perforation without septum. Teeth fl at (with round or slightly pointed tip, but never spine-like). Teeth on DP arranged as single row.

V
. Vertebrae zygospondylous; beyond segment 5 with short keel; without oral bridge.

Description
A . Arm shape unbranched; length 3-4 × disc diameter, or length more than 4 × disc diameter; covering without granules / spines / tubercles. Arm combs absent. Dorsal arm plate present. Accessory dorsal arm plate absent. Ventral arm plate present. Accessory ventral arm plate absent. Girdle of tiny hooks on the arms absent. Spines all similar in shape. Arm spine cross section round. The ratio of thickness of distal ⅓ of arm spine to maximum thickness (in a given cross-sectional plane) 0.6-0.9, or 0.9-1. Arm spines pointed; mainly lateral; predominantly erect, standing perpendicular to arm axis. Longest arm spines between half and one segment long, or between one and two segments long. Tentacle scales present; pores visible along most of the arm; slightly longer than wide; shorter than the spine nearby; cross section oval. D . Disc pentagonal or circular; high (above arm level). Integument not obscuring plates. Dorsal disc without granules; with obvious primary plates; with scales; with spines. Dorsal disc scale / plate without tubercles. Radial shield present; without granules / spines; length more than half of the disc radius; scalene (oblique) triangular.

V
. Gonads restricted to the disc. Ventral interradii without granules / spines. Scales on ventral interradii covering the whole area of an interradius. The number of scales on H line of an interradius about 14-21. Ventral teeth single. Infradental papillae on oral plate present. Buccal scales on OP present. Accessory oral papillae on OP absent. Lateral oral papillae on OP absent. Adoral shield spine present. Lyman's ossicle present. Oral shield covering less than one third of interradius; much longer than wide or about as long as wide. Madreporite one; with one pore. Adoral shield meeting in front of the oral shield. Abradial genital plate not twisted. Number of genital slits one per bursa. Genital slit longer than half the length of an interradius. Genital papillae absent. Dental plate of equal width all over, or dorsal half widest; entire (single plate). Dental sockets slit-shaped; equal to or more than 50% of the width. At least one socket on DP as perforation with septum. Teeth block-like (with square tip). Teeth on DP arranged as single row.

V
. Vertebrae zygospondylous; beyond segment 5 with short keel; without oral bridge. D . Disc circular; fl at (about same level as arms). Integument not obscuring plates. Dorsal disc with granules; with obvious primary plates; with scales; with spines. Dorsal disc scale / plate with tubercles. Radial shield present; with granules and with tubercles grown out of plate; length between one third and half of the disc radius; scalene (oblique) triangular.

V
. Gonads restricted to the disc. Ventral interradii with spines. Scales on ventral interradii absent. Ventral teeth single. Infradental papillae on oral plate present. Buccal scales on OP present. Accessory oral papillae on OP absent. Lateral oral papillae on OP absent. Adoral shield spine present. Lyman's ossicle present. Oral shield covering less than one third of interradius; much wider than long. Madreporite one; with one pore. Adoral shield meeting in front of the oral shield. Abradial genital plate not twisted. Number of genital slits one per bursa. Genital slit longer than half the length of an interradius. Genital papillae absent. Dental plate ventral half widest; entire (single plate). Dental sockets circular; equal to or more than 50% of the width. At least one socket on DP as perforation with septum. Teeth block-like (with square tip). Teeth on DP arranged as a single row on ⅔ and a cluster of tooth papillae on ⅓ of dental plate.

Description
A . Arm shape unbranched; length more than 4 × disc diameter; covering without granules / spines / tubercles. Arm combs absent. Dorsal arm plate present. Accessory dorsal arm plate absent. Ventral arm plate present. Accessory ventral arm plate absent. Girdle of tiny hooks on the arms absent. Spines all similar in shape. Arm spine cross section round. The ratio of thickness of distal ⅓ of arm spine to maximum thickness (in a given cross-sectional plane) 0.6-0.9. Arm spines pointed; mainly lateral; predominantly erect, standing perpendicular to arm axis. Longest arm spines longer than two segments long. Tentacle scales present; pores visible along most of the arm; more than two times as long as wide; as long as the spine nearby; cross section round. Position of lateral arm plate (LAP) arched (wrapped around the arm). Dorsal and / or ventral edge(s) of LAPs without constriction. Ventral position of LAP projecting ventro-proximalwards. Ventro-distal tip of LAP not projecting ventralwards. Arm spine articulation with dorsal and ventral lobes; without sigmoidal fold; arranged over entire distal LAP edge; larger in middle of arm. Distance between spine articulations dorsalwards increasing. Nerve and muscle openings separated by small ridge if at all.
Nerve opening approximately as large as muscle opening. D . Disc circular; fl at (about same level as arms). Integument not obscuring plates. Dorsal disc without granules; with concealed primary plates; with scales; with spines. Dorsal disc scale / plate without tubercles. Radial shield present; with spines; length more than half of the disc radius; scalene (oblique) triangular.

V
. Gonads restricted to the disc. Ventral interradii with spines. Scales on ventral interradii absent. Ventral teeth single or with several tooth papillae (O'Hara et al. 2018). Infradental papillae on oral plate absent. Buccal scales on OP present. Accessory oral papillae on OP absent. Lateral oral papillae on OP absent. Adoral shield spine absent. Lyman's ossicle absent. Oral shield covering less than one third of interradius; much wider than long. Madreporite one; with one pore. Adoral shield separated. Abradial genital plate not twisted. Number of genital slits one per bursa. Genital slit longer than half the length of an interradius. Genital papillae absent. Dental plate ventral half widest; entire (single plate). Dental sockets slit-shaped and circular; less than 50% of the width of to equal to or more than 50% of the width. At least one socket on DP as perforation with septum. Teeth fl at (with round or slightly pointed tip, but never spine-like). Teeth on DP arranged as a single row on half of dental plate and other half with a cluster of tooth papillae (ventral cluster).

Discussion
One of the main results of this study is the analysis of previously introduced characters (Smith et al. 1995;Pomory 2007;Thuy & Stöhr 2016;O'Hara et al. 2018) and selection of those most useful for family identifi cation. To illustrate, the dental plate was described in greater detail in Thuy & Stöhr (2016) than in Smith et al. (1995), i.e., the geometry of the dental plates, as well as the shape, depth, and size of its sockets. Moreover, the arrangement of teeth and the cluster of tooth papillae is here categorized into four groups to cover all available variations, regarding single / multiple columns and the ratio of teeth to tooth papillae for each family. In addition, dorsal and ventral covering of arm and disc are modifi ed, following the terminology described above, to better defi ne what 'scale', 'plate', 'tubercle' and 'granule' mean and how they are assessed in an organized classifi cation. Although Pomory (2007) illustrated the arm spine shapes, it was not applicable at the family level, and still some details were overlooked. We revise the classifi cation of the arm spine shapes by integrating the tip and cross-sectional shapes, to defi ne all types of arm spine, including fl at, tapering, pointed, and circular. Hendler's study (2018) was instrumental in clarifying the confusion about the development and homologies of the oral papillae, and for the fi rst time his new terminology has been applied in the current identifi cation key. Tentacle scales were termed 'podial scales' in some earlier publications (Pomory 2007) and ambiguously described with subjective terminology (Pomory 2007;O'Hara et al. 2018). Here, apart from the visibility of the tentacle pores along the arm, the tentacle scale length is described relative to the nearest spine to overcome subjective terms such as long or short. Moreover, diff erent descriptions of shape suggested earlier (Thuy & Stöhr 2016) are substituted by a comparison of the length of each tentacle scale with its width combined with the shape (oval / round) of its cross-section. The defi nition of 'keel' based on LeClair (1996) occurs only in Ophiopholis, Ophiotrichidae and Ophionereis. We use "extended keel" as an equivalent to LeClair's defi nition and "short keel" in the broader sense of an arrow-shaped projection. According to this defi nition, the families Ophiopholidae, Ophiotrichidae and Ophionereididae have an extended keel and all others with the exception of Euryalida (non-keeled) have a short keel. To conclude, our result of creating a functional key suggests that the chosen states for each family are in good agreement with those mentioned in previous studies (Mortensen 1927;Fell 1960;Clark & Rowe 1971;Paterson 1985;Pomory 2007;Martynov 2010a;Gondim et al. 2013).
The possibilities off ered by computer-based multi-access keys (interactive keys) have rarely been explored for brittle stars. Possibly the fi rst of its kind was presented by Smirnov et al. (2006), for species of the Arctic and Antarctica, but he used a specially developed software that is unfortunately not available anymore. A key to Antarctic species created with DELTA was available on the website of the Melbourne museum but can no longer be accessed. The only interactive key to brittle stars still freely available online comprises 16 species from south-eastern Brazil (Alitto et al. 2016) and was created using the software Xper (Ung et al. 2010), which is similar in functionality to DELTA. Walter & Winterson (2007) mentioned DELTA as a standard format for taxonomic descriptions and also as a database for interactive keys, despite the fact that the format is rather infl exible compared with the Structure of Descriptive Data (SDD) format, which was proposed as a standard for taxonomic descriptions by the association for biodiversity information standards, TDWG (www.tdwg.org). Coleman et al. (2010) also described the advantage of the software package DELTA for taxonomists and prepared an introduction to the software for beginners. The interactive identifi cation key (Intkey) shows an advantage over conventional keys, due to its fl exibility of applying characters in any order, interactive techniques such as automatically showing relevant images and allowing choice of states by clicking on an image, the possibility of updating the key (character deletion and change), when more data are available, and setting error tolerance and expressing uncertainty while using the intkey.
The keys presented in this study are the fi rst conventional and interactive keys for the 33 currently accepted families, albeit still based on a limited selection of examined species, and the fi rst release version still has limitations. Identifi cation to the family level is more diffi cult than species identifi cation, because many families are composed of a large number of species, usually grouped in several genera. The morphological diversity is thus considerably higher than within a species, and the delimitation of a family must be based on the selection of characteristics shared by as many species as possible. Pending the full taxonomic revision of each family, we attempted to delimit the families from one another by examination of selected species, but the (non-)availability of material (particularly of rare species) and a reasonable eff ort with highly time-consuming methods, limited the number of included species. Possibly, not all family-specifi c characters needed to correctly assign all species to a family have been recognized yet. While nomenclatural rules require that the type species of a family must be the basis of that family, the type species does not always share many characters with the majority of other included species. For example, the Ophiomyxidae as currently understood, include genera (e.g., Ophioconis) that are superfi cially more similar to Ophiodermatidae than to Ophiomyxa and were previously placed in that family. On the other hand, Ophiomusaidae and Ophiosphalmidae are separated by few characters. These limitations highlight the diffi culty of collecting data from morphological characters and interpreting the available states. Direct observation (albeit on images) is a painstaking and highly time-consuming task. However, it has been shown that keys created from published data alone, without direct observation, are less accurate (Drinkwater 2009). Nevertheless, there might be some obstacles during the observation, since some characters might not be discriminate enough with diff erent preservation methods, either wet or dried, e.g., thick / thin skin. We categorized the surface of the arm spine into fi ve diff erent states, but excluded them from our study in the end since the wet / dry preservation had an eff ect on arm spine surface shape. Technically, missing data are handled in DELTA Intkey and in conventional keys, but inadequate data can prevent the separation of some taxa. We tested the separability of taxa in Intkey (using diagnosis level 1) and the result showed that the tested taxa were separated using the current characters and chosen states. Given the above considered limitations, the result from such an analysis should of course be treated with considerable caution.
The polychotomous conventional key created by DELTA from the full dataset is limited to a single entry point and a single pathway that must be followed to identify a specimen to family level. If any of the choices are impossible, because the specimen is incomplete or lacks the character in question (which can be the case with species not included in this study), an identifi cation may not be possible. To facilitate its use in the fi eld, where computers may not be available and preparations and microscopy may be diffi cult, the conventional key was built with priority on external characters, which may make it less accurate, though, because recent studies have emphasized the taxonomic importance of internal and microscopic features (Martynov 2010a;Thuy & Stöhr 2016;O'Hara et al. 2018). In contrast, a tabular key has multiple entry points and may be a better alternative. The full data matrix can serve as a tabular key, but with 77 characters and 33 taxa, it is unwieldy. A condensed tabular key was created with DELTA as a more manageable compromise (Appendix 2).
Our approach should lend itself well for use by both general users of the key and specialist researchers. We conclude that an interactive software-based key is a practical and eff ective alternative to the conventional printed key. Further work should look into fi nding new characters and further observation of more genera from each family to construct a reliable and up to date identifi cation key. We compared two diff erent types of freely available software for the creation of identifi cation keys and conclude that both are suffi cient for this purpose, although users may favor one of them for particular features.
The family descriptions derived from this study are more detailed than the diagnoses provided by O'Hara et al. (2018), but they are still based on a small number of species and genera, and will need to be updated in the future. Some families were newly created by O'Hara et al. (2018) and consequently, they have never been described in detail before.