356 CONCLUSION. 
could, at pleasure, control its ascent to the sur- 
face, or descent to the bottom of the sea. 
To creatures that sometimes floated, a thick 
and heavy shell would have been inapplicable ; 
and as a thin shell, inclosing air, would be 
exposed to various, and often intense degrees of 
pressure at the bottom, we find a series of pro- 
visions to afford resistance to such pressure, in 
the mechanical construction both of the external 
shell, and of the internal transverse plates which 
formed the air chambers. First, the shell is made 
up of a tube, coiled round itself, and externally 
convex. Secondly, it is fortified by a series 
of ribs and vaultings disposed in the form of 
arches and domes on the convex surface of this 
tube, and still further adding to its strength. 
Thirdly, the transverse plates that form the air 
chambers, supply also a continuous succession 
of supports, extending their ramifications, with 
many mechanical advantages, beneath those 
portions of the shell which, being weakest, were 
most in need of them. 
If the existence of contrivance proves the exer- 
cise of mind ; and if higher degrees of perfection 
in mechanism are proof of more exalted degrees 
of intellect in the Author from whom they pro- 
ceeded; the beautiful examples which we find in 
the petrified remains of these chambered shells, 
afibrd evidence coeval and co-extensive with the 
mountains wherein they are entombed, attesting 
