302 CLASS V. CEPHALOPODA. ORDER I. POLYTHALAMIA. 
M. Valenciennes regards the periodical introduction of a partition in 
the shell of Nautilus as in some measure analogous to the occasional de- 
posit of a varix in the shell of Murex and other Gastropods. The septa, 
like the varices, may undoubtedly be secreted by the mantle during a 
period of rest, but there the analogy ceases. We would rather compare 
the rotatory increase of the Nautilus to the horizontal growth of that sin- 
cular Gastropod, the Magilus (vide p. 231). The one gravitates round a 
centre, increasing by a peculiarity of contrivance the volume and com- 
parative buoyancy of its shell to keep pace with the surrounding pressure, 
which naturally increases in intenseness as the subject increases in bulk ; 
the other having a different medium to combat with, namely, the outward 
increase of the coral in which it is imbedded, leaves its spiral plan of 
construction to pursue a straight growth, and, raising itself forward, fills 
the vacated portion of the shell with an extraordinary secretion of solid 
matter. If the Magilus had advanced by a deposit of transverse septa, 
instead of solidifying its shell, the increase of the madrepore might have 
crushed it; and if the Nautilus had advanced by the solidifying of its 
shell instead of by the deposit of transverse septa, it would have pro- 
duced an incumbrance incompatible with its locomotive faculties. 
We are now brought to the consideration of the habits of the Nautilus. 
It is evidently, as Mr. Owen expresses it, ‘‘ a ground-dwelling animal,” 
creeping along the bottom of the sea, with hood and tentacles, at a tole- 
rably quick pace ; and the shell, being above its head, must greatly assist 
the animal in its movements, from a tendency to float. It is not im- 
probable but that the Nautilus may use a certain hydrostatic influence 
over the branchial cavity to enable it to rise to the surface. Valenciennes 
says, “Il nage avec facilité dans le sein des eaux en faisant sortir avec force 
la grand quantité d’eau contenue dans sa cavite branchiale.” And the testi- 
mony of Rumphius in respect to its capacity of floating, cited by Mr. 
Owen, is of so much interest, considering the time in which it was writ- 
ten, that we venture to repeat it. 
““ When he thus floats on the water, he puts out his head, and all his 
barbs (tentacles), and spreads them upon the water with the poop (of the 
shell) above; but at the bottom he creeps in the reverse position, with 
oe F 
