112 PAPERS, ETO. 
lower than the blue lias. But itis not always the case 
that, where ammonites abound, the Aptychus is in equal 
abundance; on the contrary, there are many beds contain- 
ing ammonites in great numbers, in which the Aptychus 
is rarely, if ever, found. This may have been accidental, 
or depending upon the circumstance, that the character of 
the beds in which they are absent was not such as to tend 
to their preservation, as in the case of others deposited 
under more favourable circumstances. In the beds of 
the lower lias it is occasionally found symmetrically 
placed in the impressions of ammonites, having a some- 
what semicircular form, thin and wrinkled in its appear- 
ance, and of a very dark colour. In the upper lias, 
the Aptychus differs in form. It is there striated, and 
formed of two valves, somewhat resembling a bivalve shell, 
but without the hinge, which is always possessed by these 
shells. It is generally of a brown colour, and horny in its 
structure, but still occasionally shewing the deep carbon- 
aceous black tint possessed by those in the beds below. In 
the green-sand it again alters its character, and becomes 
punctured and porous in appearance. The conclusion 
that the Aptychus might belong to shells of the Cephalo- 
poda, was arrived at, from its association in the same bed 
with the ammonite, and from its being frequently found 
in its outer chamber. Now the mere circumstance of its 
being so found in the chamber of the ammonite, although 
presumptive evidence, I do not consider sufficient to 
establish the fact of its having anything to do with the 
anımal inhabiting that shell ; because we are aware that 
many parasitic creatures take possession of the shells of 
other animals and make them their place of abode. From 
my own observation, I repeatedly find this to be the case, 
and that with the Aptychus are associated many such para- 
