INTRODUCTION. 7 



scarcely to be doubted, but that many, which are 

 now universally accepted as distinct species, will 

 hereafter be discovered to be only varieties caused 

 by locality or age. The young shell is often ap- 

 parently cast in the mould of a genus very diffe- 

 rent from its own ; still there are, in general, some 

 internal or external marks, some obscure charac- 

 ter, which, if previously known and attended to, 

 will prevent the frequency of error. With a view 

 to offer some, though very imperfect, assistance 

 in this respect, a few hints are subjoined of the 

 known variations occasioned by marginal incre- 

 ment, or by superficial depositions of testaceous 

 matter. There are doubtless many more which 

 might be mentioned, and which experience will 

 point out to the careful observer better than any 

 attempt at explanation. 



One difference between young and old shells is 

 nearly universal, which is, that the former are thin- 

 ner, lighter, more transparent, and generally paler 

 in their colours. In figure the multivalves and 

 bivalves vary little, if at all : except that in som 

 genera the mode of increment is plainly discern- 



