404 



ECHIN0Z0A. 



and survive at the present day. The recent genus Ophiolepis is found 

 in the Pleistocene, and possibly occurs in Tertiary deposits also. 



Finally, there are various Palaeozoic Ophiuroids which cannot 

 be included in either of the preceding sections, and which may be 



Fig. 280. — Aspidura loricata. Muschelkalk. 



provisionally placed in a special division (Protophiurida), though 

 their characters are not completely understood. All the forms here 

 in question agree with the typical Ophiurids in having five simple 

 plated arms, but the " madreporite " is placed on the dorsal aspect 

 of the body, and the under surface of the arms exhibits a double, 



Fig. 281. — a, Outline of Eugaster Logani, of the natural size — Devonian, b, Base of an arm 

 of the same viewed from below, enlarged, c, Outline of Protaster Forbes/, of the natural size — 

 Silurian. D, Base of an arm of same, viewed from below, enlarged, e, Portion of the arm of 

 Ptilonaster princeps, viewed from below, enlarged — Devonian, a, Ventral (superambulacral ?) 

 plates ; b, Adambulacral plates ; p, Pore. (After Hall.) 



in place of a single, row of plates (fig. 281, b). Two views may be 

 held as to the nature of the double (or sometimes quadruple) row 

 of plates exhibited on the under side of the arms of the Protophiu- 

 rids. On one view, these plates are considered as being integument- 



