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Show 1896.] OF THE PALAEOZOIC OPHIUROIDEA. 1031 plates opposite one another ; they have dorsal and ventral arm-plates ; they have no ambulacral groove. To include these genera among the Lysophiurae would limit the diagnosis of that order to the single character of the uufused nature of the ambulacral plates. It is therefore advisable to retain Ophlohelus and OphiothoUa among the Streptophiurae and attribute the character of their ambulacral plates either to immaturity or degeneration. Support to this conclusion is given by the fact that even among the Zyg-ophiurae the ambulacral ossicles begin as pairs of simple free bars (Ludwig, 8. Bd. ii. p. 94, pl. x. figs. 2-5). Pamily 1. PEOTASTEEIDA Diagnosis.-Lysophiurae which have boot-shaped ambulacral ossicles. Each of the in consists of a " body " lying beside the middle line of the arm, and of a lateral " wing" projecting transversely from the body of the ossicle. Genus 1. PROTASTER, Forbes, 1849 [4. pl. iv.]. Synonyms : Protaster, Hall. Protaster pars of Salter, Billings, Miller, Gregory, Stiirtz. N o n Protaster of Meek and Worthen, Dewalque, Davy. Diagnosis.-Protasteridae with a well-marked disc; long, tapering, very flexible arms. Some of the adanibulacral ossicles are Y-shaped. Scales of the disc fairly large. Type species.-Protaster sedgwicki, Forbes, 1849. Silurian, Westmoreland. (Pig. 1«, b, c.) Fig. 1. yy ^w SV^M? qb l^p fd&4 Uy ^J ^^ a b c Protaster sedgwicki, Forbes: the structure of the arm ; a, near the distal end b, in the middle; c, at the proximal end. Remarks.-This genus was founded by Forbes on specimens from the Ludlow rocks of Kendal, Westmoreland, which are iu the Cambridge Museum1. Forbes's figures of the arm-structure are not satisfactory, and the accompanying diagrams will help to explain it. 1 I must express m y thanks to Prof. T. McKenny Hughes and Mr. H. Woods for facilities in examining the type. I a m indebted to Mr. E. T. Newton and M r H. Allen for tbe opportunity of seeing the actual mould which was studied by Forbes, aud also for that of describing the following species. |
