96 
the shell as it had then quitted. To enable it to do this, by forming 
a transverse septum of an appropriate form, it needed only to possess, 
at its posterior termination, an organization calculated for the se- 
cretion, deposition, and modelling, of shelly matter. 
The absolute weight of the animal must necessarily have increased 
with its growth ; but if with this increase of growth an additional 
chamber of air was produced, the animal and its appendage would pre- 
serve the same degree of specific gravity. It is evident, that whilst a 
due proportion was preserved between the solid part of the animal and 
these testaceous air-vessels, the animal to which they were attached 
would not be at all affected by their weight, let the number or size of 
those which were accumulated on it have been ever so considerable. 
It seems to be a characterizing property of the animals belonging 
to some of the shells of this genus, to close or fill up all that posterior 
part of the shell which they do not inhabit : and in some of these we 
have seen, that by leaving these chambers empty, the shell and animal 
have probably preserved the same degree of buoyancy through their 
whole growth. But in another very curious species, the Serpula 
heliciformis, known only in a recent state, the whole dwelling of 
which appears to be not testaceous, but actually spathous, the pos- 
terior seeming snail part is gradually filled up, so as to become a 
solid mass of apparent spathose matter. 
To account for this difference, it only seems to be required to con- 
sider that this shell does not attach itself to light bodies, whose buoy- 
ancy it would affect, but that it is always found imbedded in fixed 
masses of madrepore, and in general of Madreporci meandrites, Linn, 
where, from the body being fixed, in which it inhabits, no regulation 
of its weight is necessary. 
The fossil, PI. VII. F. 7, formed by almost horizontally disposed spiral 
turns, flatfish, but rather rounded and slightly carinated on each side, 
the last turn being a little produced, and terminating in a round and dis- 
tinctly marginated aperture, is, as well as the following , introduced in this 
