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 3 still there are, in general, some 

 internal or external marks, some obscure charac- 

 ters, 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 increment, or by 

 superficial depositions of testaceous matter. There 

 are doubtless many more which might be men- 

 tioned, and which experience will point out to the 

 careful observar better than any attempt at expla- 

 nation. 



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 some 

 genera the mode of increment is plainly discern- 



