328 
NAUTILUS POMPILIUS. 
sure through a membrane that covers the column 
of water in a tall glass, forces a portion of this 
water into the cavity, or single air-chamber of 
the balloon, which immediately begins to sink ; 
on the removal of this pressure, the elasticity 
of the compressed air causing it to return to its 
former volume, again expels the water, and the 
balloon begins to rise.* 
I shall conclude this attempt to illustrate the 
structure and economy of fossil Nautili by 
those of the living species, by 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, sup- 
posing it to be the receptacle only of a fluid 
secretion, interchanging its place alternately 
from the siphuncle to the pericardium, would 
be subsidiary to the movements of the animal, 
both at the surface, and bottom of the sea. 
First, The animal was seen and captured by 
partially, or entirely filled with pure crystallized carbonate of 
lime. (See PI. 42, Fig, 1, 2, 3, and PI. 36). In all such cases, 
it is clear that no communication existed, by which water could 
pass from the interior of the siphon to the air chambers. When 
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. 
* The substance of the siphuncle is a thin and strong mem- 
brane, with no appearance of muscular fibres, by which it could 
contract or expand itself; its functions, therefore, must have 
been entirely passive, in the process of admitting or ejecting any 
fluid to or from its interior.— See Owen’s Memoir, p. 10. 
