216 
Fig. 3, seems to agree with 0. deformis, of Lamarck. Its irregular 
and deformed figure is not capable of being described. It is from one 
to two inches in length, and half as broad. The under valve is much 
more ventricose than the upper. 
The oyster, the lower valve of which is represented Plate XIY. Fig. 
5, is very remarkable. It is rather more than an inch and a quarter in 
length, and nearly an inch in breadth. Its base is transversely extended 
and truncated, so as to form a straight line : the cartilaginal pit, which 
is trigonal and very obliquely disposed, somewhat resembles that of a 
Pecten : at the same time, that an obtuse, tooth-like ridge, lying ob- 
liquely across the hinge -pit, gives it much of the appearance of a 
Chama. This shell very much accords with 0. biauriculata, Lam. 
The fossil oyster represented Plate XIY. Fig. 16, is highly interesting. 
This minute shell, of which I possess two valves, one only of which is 
perfect at the hinge, is from Verona. It but little exceeds half an inch 
in length, and three-eighths of an inch in width. The cartilaginal pit 
extends high up into the shell, and is very finely striated. The circum- 
stance which most particularly claims attention in this curious little 
fossil, is, that its thickness is nearly a third of its length, and that its 
substance is composed of a striated spar, the columnar crystals forming 
which are disposed perpendicularly to the plane of the shell. Other in- 
stances of this striated structure, in fossil shells, have been already shown. 
A fossil oyster before me, from the neighbourhood of Maidstone, 
in Kent, is remarkable for the great depth of its under valve, which 
is nearlyas deep as the shell is long ; being two inches in depth, and 
only three in length. 
Having now placed before you such of the fossil oysters, with either 
a smooth or irregularly rough surface, as appeared to me to be the 
most interesting, I shall now call your attention to those fossil oysters, 
the surfaces of which are regularly plicated. These I shall divide into, 
1st, those which have somewhat of a roundish form, and in which the 
plicae radiate from the beak to the circumference of the shell; and, 2dly, 
