1896. ] OF THE PALZHOZOIC OPHIUROIDEA, 1031 
plates opposite one another; they have dorsal and ventral arm- 
plates; they have no ambulacral groove. To include these genera 
among the Lysophiure would limit the diagnosis of that order to 
the single character of the unfused nature of the ambulacral plates. 
It is therefore advisable to retain Ophiohelus and Ophiotholia 
among the Streptophiure 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- 
ophiure the ambulacral ossicles begin as pairs of simple free bars 
(Ludwig, 8. Bd. ii. p. 94, pl. x. figs. 2-5). 
Family 1. PROTASTERID2. 
Diagnosis.—Lysophiure which have boot-shaped ambulacral 
ossicles. Each of them 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. Prorasrmr, Forbes, 1849 [4. pl. iv.]. 
Synonyms : 
Protaster, Hall. 
Protaster pars of Salter, Billings, Miller, Gregory, Stiirtz. 
Non Protaster of Meek and Worthen, Dewalque, Davy. 
Diagnosis. —Protasteride with a well-marked disc; long, tapering, 
very flexiblearms. Some of the adambulacral ossicles are Y-shaped. 
Scales of the disc fairly large. 
Type species—Protaster sedgwicki, Forbes, 1849. Silurian, 
Westmoreland. (Fig. la, }, c.) 
LM by Q legge 
ra) Ed SC 
~~ 
VIZ J pt Woe 
a b Q 
Protaster sedgwicki, Forbes: the structure of the arm ; a, near the distal end ; 
6, in the middle; ¢, at the proximal end. 
Remarks.—This genus was founded by Forbes on specimens 
from the Ludlow rocks of Kendal, Westmoreland, which are in 
the Cambridge Museum’. Forbes’s figures of the arm-structure are 
not satisfactory, and the accompanying diagrams will help to 
explain it. 
1 T must express my thanks to Prof. T. McKenny Hughes and Mr. H. Woods 
for facilities in examining the type. I am indebted to Mr. HE. T. Newton and 
Mr. H. Allen for the opportunity of seeing the actual mould which was studied 
by Forbes, and also for that of describing the following species. 
