OPHIUROIDEA. 



257 



genus Eugastcr (fig. 148, A and B) the structure is essentially 



the same as in Protaster, but the disc is prolonged along the 



bases of the arms, and the plates of the disc are articulated 



by their edges, and do not overlap. In the Devonian Ptilo- 



naster (fig. 148, E), again, we have a form fundamentally 



similar to the preceding, but having four rows of perforated 



plates on the lower surface of the arms. The chief question 



as to the affinities of the types just alluded to turns upon 



the true nature of the perforated plates, in double or quad- 



ruple series, placed along the lower surface of the arms. 



These have generally been regarded as identical with the 



" ventral plates " of the 



arms of the ordinary 



Ophiuroids structures 



which belong wholly to 



the integumentary skel- 



eton, and which are not 



represented in the As- 



teroids. It does not 



seem certain, however, 



that this is the true na- 



ture of the perforated 



plates of the under sur- 



face of the arms of the 



genera above alluded to; 



and it is not impos- 



sible that they are really 



the internal " ambula- 



cral ossicles " of the 



arms, exposed to view 



by the destruction Of 



tVip cmnprfipinl " vpntral 

 Ventral 



plates " during fossilisa- 

 tion. Should this turn out to be the case, the structure of 

 Protaster and its allies is not so abnormal. If, on the other 

 hand, these perforated plates are really the " ventral plates," 

 then these genera exhibit the peculiarity that the under sur- 

 faces of the arms are pierced by " ambulacral pores." 



The only other Palaeozoic type which needs notice here is 



VOL. I. R 



Fig. U7.- Protaster SedgwictH. Upper Silurian. 



A> Disc and bases of the anns ' ma 8 nified '> B > 

 of an arm g^^ enlarged. 



