348 VARIED PKOI'ORTIONS OF SUPPORT. 
The shell of the Ammonites Heteropliyllus 
(PL 08, and PI. 39,) affords beautiful exemplifi- 
cations of the manner in which the mechanical 
strength of each transverse plate is so disposed, 
as to vary its support in proportion to the dif- 
ferent degrees of necessity that exist for it in 
different parts of the same shell.* 
titions of an Ammonite. PI. 35 represents the form of the exter- 
nal shell, wherein the body occupied the space extending from 
a. to c, and corresponding with the same letters in PI. E. 36. 
This species has a single series of strong ribs passing obliquely 
across the shell of the outer chamber, and also across the air- 
chambers. From c. to the inmost extremity of the shell, these 
ribs intersect, and rest on the sinuous edges of the transverse 
plates which form the air chambers. These edges are not seen 
where the outer shell is not removed. (PI. 35, e.) A small 
portion of the shell is also preserved at PI. 35, b. 
From d. inwards, these sinuous lines mark the terminations of 
the transverse plates at their junction with the external shell ; 
they are not coincident with the direction of the ribs, and there- 
fore more effectually co-operate with them in adding strength to 
the shell, by affording it the support of a series of various props 
and buttresses, set nearly at right angles to its internal surface. 
* Thus on the back or keel, PI. 39, from V. to B., where the 
shell is narrow, and the strength of its arch greatest, the intervals 
between the septa are also greatest, and their sinuosities com- 
paratively distant ; but as soon as the flattened sides of the same 
shell, PI. 38, assume a form that offers less resistance to external 
pressure, the foliations at the edges of the transverse plates 
approximate more closely; as in the flatter forms of a Gothic 
roof, the ribs are more numerous, and the tracery more com- 
plex, than in the stronger and more simple forms of the pointed 
arch. 
The same principle of multiplying and extending the ramifica- 
tions of the edges of the transverse plates, is applied to other 
species of Aunnonites, in which the sides are flat, and require a 
