238 



PROCEEDINGS COTTESWOLD CLUB 



1911 



Gervillia and Perna range upwards from the Trias, and 

 where they are first found in the Lias aheady possess well- 

 marked generic characters. Inoceramus, on the other hand, 

 makes its first appearance in the Lower Lias, and some of the 

 forms which occur at that horizon differ somewhat from the 

 typical examples of the genus which are found in Cretaceous 

 rocks. Some of them exhibit a strong resemblance to Perna, 

 which is probably the parent-genus. 



Owing to the fact that the classification of this group of 

 shells is based primarily on the characters of the cardinal area, 

 it is frequently a matter of difficulty to assign any particular 

 form to its proper genus. For it is only in exceptional cases 

 that the hinge-area is exhibited, and when it is not, the general 

 form of the shell has to be used as a guide. 



Below is given, in tabular form, a summary of the chief 

 characters of each of the three genera, Gervillia, Perna and 

 Inoceramus. 



