344 TRANSVKESE PLATES AND AIR CHAMBERS. 
dantly beneath those parts only, which without 
them, would have been weak. 
We find an infinity of variations in the form 
and sculpture of the external shell, and a not 
less beautiful variety in the methods of internal 
fortification, all adapted, with architectural ad- 
vantage, to produce a combination of Ornament 
with Utility. The ribs also are variously multi- 
plied, as the increasing space demands increased 
support ; and are variously adorned and armed 
with domes and bosses, wherever there is need 
of more than ordinary strength. 
Transverse Plates, and Air Chambers. 
The uses of the internal air chambers will best 
be understood by reference to our figures. PI. 
36 represents a longitudinal section of an Am- 
monite bisecting the transverse plates in the 
central line Avhere their curvature is most simple. 
On each side of this line, the curvature of these 
plates become more complicated, until, at their 
termination in the external shell, they assume 
a beautifully sinuous, or foliated arrangement, 
resembling the edges of a parsley leaf, (PI. 38), 
the uses of which, in resisting pressure, I shall 
further illustrate by the aid of graphic repre- 
sentations. 
At PI. 35, from d. to e. we see the edges 
of the same transverse plates which, in PI. 36, 
