FORTIFICATION OF CHAMBERS. .'523 
First, the circumference of tJie external shell 
is constructed every way upon the principles of 
an Arch (see PI. 31, Fig. 1, and PI. 32, Fig. 1.), 
so as to offer in all directions the greatest resist- 
ance to any pressure that tends to force it inwards. 
Secondly, this arch is further fortified by the 
addition of numerous minute Ribs, which are 
beautifully marked in the fossil specimens repre- 
sented at PL 32, Fig. 1 . In this fossil the external 
shell exhibits fine 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 PL 32, Fig. 1. 
a. to b.) 
Thirdly, the arch is rendered still stronger by 
the disposition of the edges of the internal Trans- 
verse plates, nearly at right angles to the sides 
of the external shell, (see PL 32, Fig. I, 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 
pressure of deep w^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 
an extraordinary number of transverse beams 
and bulk heads.* 
* The disposition of the curvatures of the transverse ribs, or 
lines of growth, in a different direction from the curvatures of 
the internal transverse plates, affords an example of further con- 
trivance for producing strength in the shells both of recent and 
