ILLUSTRATED BY NAUTILUS POMPILIUS. 317 
with a preliminary knowledge of the facts, that 
the recent shells, hoth of N. Poinpilius and 
Spirula, are formed by existing Cephalopods ; 
and we hope, through them, to be enabled to 
illustrate the history of the countless myriads of 
similarly constructed fossil shells whose use and 
office has never yet been satisfactorily explained. 
We may divide these fossils into two distinct 
classes ; the one comprising external shells, 
whose inhabitants resided, like the inhabitant 
of the ISr. Pompilius, in the capacious cavity of 
their first or external chamber (PI. .31, Fig. 1) ; 
the other, comprising shells, that were wholly 
or partially included within the body of a Cepha- 
ffipod, like the recent Spirula, (PI. 44, Figs. 1, 2). 
In both these classes, the chambers of the shell 
nppear to have performed the office of air vessels, 
or floats, by means of which the animal Avas 
enabled either to raise itself and float near the 
surface of the sea, or sink to the bottom. 
It will be seen by reference to PI. 31, Fig 1,*' 
that in the recent Nautilus Pompilius, the only 
organ connecting the air chambers, with the 
l>ody of the animal, is a pipe, or siphimcle, which 
descends through an aperture and short project- 
* The animal is copied from PI. 1 of Mr. Owen’s Memoir; the 
®hell from a specimen in the splendid and unique collection of 
friend W. ,1. Broderip, Esq., by whose unreserved communi- 
cations of his accurate and extensive knowledge in Natural His- 
fC'y, I have been long and largely benefited. 
