224 DISTINCTIONS BETWEEN UPPER AND LOWER SILURIAN ROCKS. 



as already stated, much expanded, and of considerable dimensions ; and in Pembroke- 

 shire, where they contain large masses of good limestone, they are of still greater 

 breadth. 



The organic remains are described in subsequent chapters. Most of the conchifers 

 and mollusks found in these nags, occur also in the Caradoc sandstone. Certain tri- 

 lobites, however, as above stated (Asaphus Buchii and A. tyrannus), are peculiar to this 

 deposit, while other crustaceans of the genus Trinucleus, though exclusively character- 

 istic of the Lower Silurian group, are common to both the lower formations. 



Enough has already been stated to demonstrate, that as we descend into older strata, 

 we meet with fresh types of animals : for although two or three species of shells of the 

 Upper Silurian Rocks may be detected in the Lower Silurian, the mass of organic 

 remains in each group is very distinct. The least practised observer may convince 

 himself of the truth of this assertion, by comparing the figures represented in Plates 4 

 to 18, with those from 19 to 27 inclusive. In the former are included all the known 

 fossils of the Upper Silurian, in the latter those of the Lower Silurian rocks. The 

 fossiliferous distinction is indeed most strikingly maintained by the crustaceans, for as 

 yet I am acquainted with no example in which a trilobite of one group has been detected 

 in the other. 



