GASTROPODA. 759 



forms still belonging to the holostomatous Prosobranchiates and the 

 Pteropods. The latter, however, are now in part represented by 

 such normal forms as Styliola. In the Carboniferous rocks, further, 

 appear the first representatives of the Pulmonate Gastropods {Z071- 

 ites, Physa, &c.) 



In the Trias the Gastropodous fauna has, in the main, a Palaeo- 

 zoic facies, but unquestionable siphonostomatous Prosobranchiates 

 appear here (Cerithiwn, Purfiurina, 8zc.) The peculiar Palaeozoic 

 Pteropods (with the exception of Conularid) have now disappeared. 

 In the Jurassic rocks a further change occurs, and a marked develop- 

 ment of the siphonostomatous Prosobranchiates now takes place. 

 The first Opisthobranchs appear at this stage, and we meet here 

 with a number of existing genera of fresh-water Gastropods (Plan- 

 orbis, Paludina, Melania, &c.) In the Cretaceous rocks the siphon- 

 ostomatous Prosobranchiates continue to increase in number, and 

 in the Tertiary period they become the predominant group, the 

 Gastropods as a whole becoming at the same time the predominant 

 group of the Mollusca, a condition of things which still subsists at 

 the present day. 



