008 SUPPLEMENTARY NOTES. 
material retardation. The part of the shell also which is foremost 
in all the retrograde movements of the animal, in the act of ascend- 
ing and descending, and also in swimming at the surface, is that 
which receives the least resistance from the fluid through which 
it moves, and at the same time presents the strongest part or back 
of the shell to any body against which it may strike, either when 
floating on the surface, or on arriving at the bottom of the sea. 
P. 331. Mr. Owen observes, that the Hood, or flattened mus- 
cular disk of the Nautilus Pompilius, seems calculated to act as 
the chief locomotive organ in creeping at the bottom ; and in the 
supine position of the animal, bears considerable analogy to the 
foot of a Gasteropod ; in a state of rest and retraction it would 
serve, like an operculum, as a rigid defence at the outlet of the 
shell. (See Owen on the Pearly Nautilus^ p. 12.) The animal 
may also assist its movements along, and adhesion to the bottom, 
by some of its numerous tentacula. 
P. 332 f. In the case of animals possessing a siphuncle and 
chambered shell, but having no means to fill the siphuncle with 
pericardial fluid, the admission and abstraction of any other se- 
creted fluid, or of water, to and from the siphuncle, would have 
a similar effect to that of the pericardial fluid of the Nautilus, in 
varying the specific gravity. It may perhaps be shewn hereafter, 
that in some of these genera an organization exists fitted to fill and 
empty the siphuncle by other agency than that of the Pericardium, 
and possibly with water admitted from the branchial cavity ; 
but as we know that the Nautilus Pompilius possesses in its pe- 
ricardial fluid and siphuncle a sufficient apparatus to effect this 
purpose, and thereby to cause the rising and sinking of this 
animal ; and as we find in the Ammonites and many extinct 
families of fossil chambered shells, a siphuncle and air cham- 
bers, very similar to those of the Nautilus; we may infer from 
analogy, that mechanisms so similar, as to those parts which 
have escaped destruction, were connected with soft and perishable 
parts, corresponding with the pericardial apparatus in the living 
Nautilus. 
It is of little importance, however, to the statical theory of 
