1896.] OF THB PALiEOZOIC OPHITJEOIDBA. 1031 



plates opposite one another ; they hare dorsal and ventral arm- 

 plates ; they have no ambulacral groove. To include these genera 

 among the Lysophiurce 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 Ophiolielus and Ophiotholia 

 among the Streptophiurte 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 2yg- 

 ophiuroB the ambulacral ossicles begin as pairs of simple free bars 

 (Ludwig, 8. Bd. ii. p. 94, pi. x. figs. 2-5). 



Pamily 1. P ii o t a s t b ii i d Ji. 



Diagnosis. — Lysoj)hiura3 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. Peotaster, Porbes, 1849 [4. pi. iv.]. 



Synonyms : 



Frotaster, Hall. 



Protaster pars of Salter, Billings, Miller, Gregory, Stiirtz. 

 Non Protaster of Meek and Worthen, Uewalque, Da\y. 

 Diagnosis. — Protasteridaj with a well-marked disc ; long, ta])ering, 

 very flexible arms. Some of the adambulacral ossicles are Y-shaped. 

 Scales of tiie disc faii'ly large. 



Type species. — Protaster sedgiviclci, Forbes, 1849. Silurian, 

 Westmoreland. (Pig. 1«, &, c.) 



Fig. 1. 

 MM (!>=:^fc=,<:^ Sv,_yl6s?' 



qb \^ Mu 

 Ugj ^-^^n ^J^<^ 



Protaster salgwiclci^ Forbes : the structure of the arm ; a, near the distal end ; 

 h, in the middle ; c, at the proximal end. 



llemarlcs. — This genus was founded by Porbes on specimens 

 from the Ludlow rocks of Kendal, Westmoreland, which are in 

 the Cambridge Museum'. Porbus's figures of the arm-structure are 

 not satisfactory, and the accompanying diagrams will help to 

 explain it. 



' I must express my thanks to Prof. T. McKenny Hughes and Mr. II. Woods 

 for facilities in examining the type. I am indebted to Mr. E. T. Newton and 

 Mr. II. Allen for the opportunity of seeing the actual mould wliich was studied 

 by Forbes, and also for that of describing the following species. 



