ILLUSTRATED BY NAUTILUS POMPILIUS. 317 
with a preliminary knowledge of the facts, that 
the recent shells, both of N. Ponipilius 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 ex- 
plained. 
We may divide these fossils into two distinct 
classes ; the one comprising external shells, 
whose inhabitants resided, like the inhabitant 
of the N. 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- 
lopod, like the recent Spirula, (PL 44, Figs. 1, 2). 
In both these classes, the chambers of the shell 
appear to have performed the office of air vessels, 
or floats, by means of which the animal was 
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 
body of the animal, is a pipe, or siphuncle, which 
descends through an aperture and short project- 
ing tube (y) in each successive transverse plate, 
* The animal is copied from PI. 1. of Mr. Owen's memoir ; the 
shell from a specimen in the splendid and unique collection of 
my friend W. J. Broderip, Esq., by whose unreserved communi- 
cations of his accurate and extensive knowledge in Natural His- 
tory, I have been long and largely benefitted. 
