Mr. Dillwyn on fossil shells. 4*7 
latitude, required a more perfect protection either from their 
enemies, or from the surrounding element, than afterwards 
became necessary. 
The same gradual approximation towards recent shells, 
which may be traced in the older strata, is also carried 
on through the tertiary formations, and the affinity, which is 
complete with respect to orders in secondary beds above the 
lias, becomes further extended, and every tertiary shell may 
be referred to some existing genus ; but though the ap- 
proximation has proceeded thus far in the London clay, yet 
all its immensely numerous species are now extinct ; and it is 
only in those uppermost beds of crag, which lie between the 
London clay and our present creation, that any fossil can be 
completely identified with a living species : the shells which 
may be thus identified are however mixed with many 
extinct species ; and though the fossils of the crag appear 
generally to have belonged to a warmer climate than ours, 
yet their character is much less tropical than those of the 
London clay, and in every respect they all approach nearer 
to the present inhabitants of the British coasts. 
I have already observed, that the shells of unknown 
families are confined to the beds below the lower oolite ; 
and in all the upper formations a relationship is completed 
between fossil and recent shells in the following regularly 
approximating series. In the secondary strata above the 
lias as to natural orders , in the London clay as to genera , and 
partially as to species in the crag. 
These observations refer exclusively to the animals of the 
9th, 10th, 11th, 12th, and 13th classes of invertebrata in 
Lamarck’s arrangement; and whether the same sort of 
MDCCCXXIV. 3 H 
