CONCLUSIONS. 269 



As the animal increased in bulk, and advanced along the 

 outer chamber of the shell, the spaces left behind it were 

 successively converted into air-chambers, simultaneously 

 increasing the power of the float. This float, being regu- 

 lated by a pipe, passing through the whole series of the 

 chambers, formed an hydraulic instrument of extraordinary 

 delicacy, by which the animal could, at pleasure, control 

 its ascent to the surface, 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, en- 

 closing 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 mechani- 

 cal construction both of the external shell, and of the in- 

 ternal 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 farther addmg 

 to its strength. Thirdly, the transverse plates that form the 

 air-chambers, supply also a continuous succession of sup- 

 ports, extending their ramifications, with many mechanical 

 advantages, beneath those portions of the shell whicii, being 

 weakest, were most in need of them. 



If the existence of contrivance proves the exercise of 

 mind ; and if higher degrees of perfection in mechanism are 

 proof of more exalted degrees of intellect in the Author from 

 whom they proceeded ; the beautiful examples which we 

 find in the petrified remains of these chambered shells, afford 

 evidence coeval and coextensive with the mountains where- 

 in they are entombed, attesting the Wisdom in which such 

 exquisite contrivances originated, and setting forth the Pro- 

 vidence and Care of the Creator, in regulating the structure 

 of every creature of his hand. 



23* 



