FOSSIL MOLLUSC A BIVALVES AND UNIVALVES. 245 



four genera are met with, among which are the ancient 

 forms of Pleiirotomaria and Euomphalus. 



But neither bivalves nor univalves are very abun- 

 dant in our British Cambrian rocks. Perhaps the 

 district of Ty-Obry, North Wales, is one of the best 

 for rinding this class of fossils, among which are 

 Palcearca socialis, and species of Ctenodonta. These 

 are, in reality, very highly 

 organized bivalves, and the 

 fact that they are among 

 the oldest known indicates, Fig " >*-***>*'* *<- 

 if evolution be true, that many other simpler forms 

 must have preceded them. 



In the Silurian rocks the true mollusca grow more 

 abundant ; and in the Upper Silurian, although there 

 are few species, these are individually abundant. 

 Both in the Cambrian and Silurian rocks (except in 

 the limestones of the latter) the fossils exist as casts. 

 The lime of the true shells has long been dissolved 

 away, and only the impressions left in the rocks 

 where they were originally imbedded. These im- 

 pressions have often been filled in by infiltration of 

 some other material, so that natural casts of them 

 have thus been taken. Nevertheless, vast numbers 

 of the fossils in these older rocks are only to be met 

 with as impressions. Many of them are very clear 

 and beautiful ; others are not only obscure, but often 

 pulled out of shape by having partaken in the mole- 

 cular movements resulting in " cleavage." 



