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- 
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 observer 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- 
