TRANSACTIONS OF SECTION C. 



389 



layer. In the case of Cephalopoda and Lamellibranchiata in the living shell 

 there are two and three layers : hence it is inconceivable the above usage should 

 have become general in the face of this knowledge. As a result of recent 

 examinations of the main genera of fossil Mollusca, the following table may be 

 given, showing the number of layers and their mineral state, for comparison with 

 living genera as given by Sorby 1 : — ■ 



Owing to the common occurrence of shells in the form of casts, or with the 

 aragonite layer transformed into calcite, or unaffected, several states occur which, 

 in writing of them for comparative purposes, need separate terms for sake of 

 conciseness and reference. These are given in tabular form : — 



The rare preservation of aragonite as shell substance in rocks of Jurassic 

 age is borne out by examination of typical types. Though of more frequent 

 occurrence in some localities in Tertiary times it is of still less frequent occur- 

 rence in the intervening Cretaceous period, when calcite formation was more 

 prevalent. 



It is hoped the distinction here made for the first time between the different 

 layers of the Molluscan shell, along with the difference in the mineral characters 

 of each, may contribute to a better understanding of the biological significance 

 of the different layers. 



1 Q.J. U.S., 1879. 



