34() CURVATURES OF TRANSVERSE PLATES. 
agree that they existed at great depths, *'daiis 
les grandes profondeurs des mers."* 
Here again we find the inventions of art 
anticipated in the works of nature, and the same 
principle applied to resist the inward pressure of 
the sea upon the shells of Ammonites, that an 
engineer makes use of in fixing transverse stays 
beneath the planks of the wooden centre on 
which he builds his arch of stone. 
The disposition of these supports assumes 
throughout the family of Ammonites a different 
arrangement from the more simple curvature of 
the edges of the transverse plates within the 
shells of Nautili ; and we find a probable cause 
for this variation, in the comparative thinness of 
the outer shells of many Ammonites ; since this 
external weakness creates a need of more inter- 
nal support under the pressure of deep water, 
than was requisite in the stronger and thicker 
shells of Nautili. 
This support is efiected by causing the edges 
of the transverse plates to deviate from a simple 
some of them empty, and others containing a fluid. The empty 
bottles were sometimes crushed, at other times, the cork was 
forced in, and the bottle returned full of sea water. The cork of 
the bottles containing a fluid was uniformly forced in, and the 
fluid exchano;ed for sea water ; the cork was always returned to 
the neck of the bottle, sometimes, but not always, in an inverted 
position. 
* See Lamarck, who cites Bruguieres with approbation on this 
point. — Animaux sans: Vert: vol. vii. p. 635. 
