FORTIFICATION OF CHAMBERS. 323 
more than air, and must consequently be ex- 
posed to great pressure when at the bottom of 
the sea. Several contrivances are therefore in- 
troduced to fortify them against this pressure. 
First, the circumference of the external shell 
IS constructed every way upon the principles of 
3^0 Arch, (see PI. 31, Fig. 1, and PI. 32, Fig. 1.) 
So as to offer the greatest resistance to pressure 
tending to force it inwards in all directions. 
Secondly, this arch is further fortified by the 
addition of numerous minute Ribs, which are 
^beautifully marked in the fossil specimens repre- 
sented at PI. 32, Fig. 1 . In this fossil the external 
shell exhibits tine wavy lines of growth, which, 
though individually small and feeble, are col- 
lectively of much avail as ribs to increase the 
Aggregate amount of strength. (See PI. 32, Fig. 1 . 
a. to b.) 
Thirdly, the arch is rendered still stronger by 
the position of the edges of the internal Trans- 
verse plates, nearly at right angles to the sides 
of the external shell, (see PL 32, Fig. 1, b. to c.) 
The course of the edges of these transverse plates 
beneath the ribs of the outer shell is so directed, 
that they act as cross braces, or spanners, to 
fortify the sides of the shell against the inward 
Pb’essure of deep u^ater. This contrivance is 
analogous to that adopted in fortifying a ship 
for voyages in the Arctic Seas, against the 
pressure of ice-bergs, by the introduction of 
