OPHIUROIDEA. 205 



the Lysophiurae and Ophioteresis), the sharp differentiation of 

 the arms from the disc, the absence from the arms of any 

 prolongation of the alimentary system, and the ventral position 

 of the madreporite. Lastly the free larva has the pluteus form. 

 The closure of the ambulacral groove and the presence of an 

 epineural canal is a feature of some importance and one 

 which the class has in common with Echinoids and Holo- 

 thurians. 



The Ophiuroidea are divided into four orders : (1) the Lyso- 

 phiurae, which are palaeozoic forms in which the ambulacral 

 ossicles alternate ; (2) the Zygophiurae (brittle-stars) in which the 

 system of dermal plates is well developed and in which the arms 

 cannot be rolled up ; (3) the Streptophiurae, which are also 

 brittle-stars, but which approach in some of their characters the 

 Cladophiurae ; (4) the Cladophiurae (gorgon-heads) which have 

 a thick integument with granular deposits without regular 

 dermal plates. 



Order 1. LYSOPHIURAE. 



The ambulacral ossicles are alternate and are not united into pairs, 

 but those of each segment are separate. There are no ventral arm plates. 

 All extinct, Silurian and Devonian. Eophiura, Bohemura, Sympterura, 

 Protester, Bundenbachia, Sturtzura, Eugaster, Ptilonaster, Taeniura, Palaeo- 

 phiura. 



Order 2. ZYGOPHIURAE (OPHIURAE). Brittle-Stars. 



The surfaces by which the ambulacral ossicles of the arms articulate 

 with one another are provided with processes and pits which fit into one 

 another and limit the movement of the ossicles upon one another. Upper, 

 under and lateral arm plates are present, and the arms are incapable 

 of coiling round straight rods. The lateral arm-plates bear spines. 



1. With arm-spines short, parallel to the long axis of the arms. 



Fam. 1. Ophiodermatidae. With numerous oral papillae, without 

 dental papillae, with arm incisions on the disc. Ophioderma M. and T., 

 Ophioncus Ives, Ophiogona Stud., Pectinura Forbes, Opliiopezella Ljg., 

 Ophiopinax Bell, Ophiopeza Ptrs., Ophiopyren Lym. (Fig. 141), Ophioconis 

 Ltk. 



Fam. 2. Ophiolepidae. With 3-6 oral papillae of which the innermost 

 is rarely infradental, without dental papillae, with arm incisions on the 

 disc. Ophiotrochus Lym. ; Ophiopaepale Ljg. ; Ophioceramis Lym. ; 

 Ophiothyreus Ljg. ; Ophiolepis M. and T. ; Ophioplocus Lym. ; Ophio- 

 zona Lym. ; Ophioplinthus Lym. ; Ophiolipus Lym. ; Ophiernus Lym. ; 

 Ophiophyllum Lym. ; Ophiochaeta Ltk. ; Ophiopleura Dan. ; Ophiopyrgus 

 Lym. ; Ophiomastus Lym. ; Ophiomusium Lym. (Fig. 143), no tentacle 

 pores beyond the basal arm-joints ; Ophiotypa Khlr. ; Ophiura Lm. ; 



