41© Mr. Dillwyn on fossil shells. 
ascertain, they have been discovered in those beds only of 
the lias, lower oolite, and chalk, which contain the larger 
ammonites. From the greater tenuity of these beaks in 
the smaller species, they may probably have yielded to 
pressure, and decay before the mud which filled them had 
become sufficiently hard to retain their shapes ; and as the 
lower mandibles of the cephalopoda are always much larger 
and thicker then the upper ones, the non-appearance of any 
of the latter may be accounted for in the same way. The 
sepias are moreover furnished with one of those thick dorsal 
plates which are commonly called cuttle-fish bones, and 
most, if not all the other sepiadas, contain an internal horny 
substance of the same nature, which is generally at least as 
thick and durable as the mandibles ; and if the fossil beaks 
of the secondary strata belonged to this family, then, in all 
probability, some of the dorsal plates would be found with 
them ; but nothing of the kind has been discovered in any 
older British stratum than the London clay. So far from 
being able to detect any traces of the naked mollusca, I have 
not been able to find, in the secondary strata, either of those 
shells by which the animal is only partially covered, nor 
any of those of the convolute, which necessarily change their 
shells at different periods of their growth, and of which the 
animal must therefore occasionally remain exposed, till a 
fresh coat of calcareous matter has been secreted. In my 
former Letter I have stated, that all the marine spiri valves of 
the secondary strata belong to operculated genera, and 
these observations serve still more strikingly to prove that, 
till the chalk deposits were completed, the mollusca, in our 
