328 NAUTILUS I'OMPILIUS. 
loon, and thus increasing its specific gravity, 
causes it to sink ; when the pressure is removed, 
the air within the chamber expands and expels 
the water, the specific gravity of the balloon is 
diminished, and it again rises.* 
I shall conclude this attempt to illustrate the 
structure and economy of fossil Nautili by 
those of the living species, with shewing in what 
manner the chambers of the pearly Nautilus, 
supposing them to be permanently filled only 
with air, and the action of the siphuncle,t sup- 
posing it to be the receptacle only of a fluid, 
interchanging its place alternately from the si- 
phuncle to the pericardium,! would be subsidiary 
to the movements of the animal, both on the sur- 
face, and at the bottom of the sea. 
First, The animal captured by Mr. Bennett, 
was seen floating at the surface, with the upper 
portion of the shell raised above the water and 
kept in a vertical position by means of the 
included air (see PL 31, Fig. 1.); this position 
the pipe was ruptured, or the external shell broken, the earthy 
sediment, in which such broken shells were lodged, finding 
through these fractures admission to the air chambers, has filled 
them with clay, or sand or limestone. 
* See Sup. Note. 
-j- The substance of the siphuncle is a thin and strong mem- 
brane, surrounded by a coat of muscular fibres, by which it could 
contract or expand itself, in the process of admitting or ejecting 
any fluid to or from its interior. (See Owen's Memoir, p. 10.) 
In our first edition it was stated erroneously that the siphuncle 
had no appearance of muscular fibres. 
\ See Sup. Note. 
