OPHIUROIDEA. 



40: 



of a small disc, and of five simple round arms, which are covered 

 with a granulated integument, and carry spines on their ventral 

 aspect. As in the living Euryalids, the arms could be rolled up 

 towards their ventral side at their tips. 



The second section of the living Ophiuroids — that of the Ophiu- 

 rida — comprises the typical forms of the order, and is characterised 

 by the fact that the arms are always simple and are protected by four 

 rows of integumentary plates (fig. 279). The integument of the disc 



;. 279. — Under surface of Ophioderma {Ophioglypjia ?) Gaveyi, of 

 Jurassic (Lias). (After Wright.) 



the natural size. 



is soft, or is covered with granules or plates of carbonate of lime. 

 The earliest types of this group appear in the Lower Devonian 

 ( Ophiurella primigenia, Stiirtz) ; and numerous forms are found in 

 the Secondary and Tertiary rocks. A well-known Triassic genus is 

 Aspidura ( = Acroura), the remains of which are widely distributed 

 in the Muschelkalk (fig. 280). The Jurassic Ophiuroids belong 

 principally to the genera Ophioderma (fig. 279), Ophioglypha, and 

 Geocoma, of which the last is peculiar to the Jurassic rocks, while 

 the two former are represented in the Cretaceous and Tertiary rocks, 



