262 SINUOUS EDGES OF TRANSVERSE PLATES. 



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 be- 

 neath 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 NautiH ; 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 internal support under the pressure of deep 

 water, than was requisite in the stronger and thicker shells 

 of NautiH. 



This support is effected by causing the edges of the trans- 

 verse plates to deviate from a simple curve, into a variety 

 of attenuated ramifications and undulating sutures. (See 

 PI. 38. and PI. 37, Figs. 6, 8.) Nothing can be more beau- 

 tiful than the sinuous windings of these sutures in many spe- 

 cies, at their union with the exterior shell; adorning it with 

 a succession of most graceful forms, resembling festoons of 

 foliage, and elegant embroidery. When these thin septa are 

 converted into iron pyrites, their edges appear like golden 

 filigrane work, meandering amid the pellucid spar, that fills 

 the chambers of the shell.* 



* The A. Heterophyllus, PI. (38,) is so called from the apparent occur- 

 rence of two different forms of foliage; its laws of dentation ai'e the same 

 as in other Ammonites, but the ascending secondaiy saddles (Pi. 38. S. 

 S.) which, in all Ammonites are round, are in this species longer than ordi- 

 nary, and catch attention more than the descending points of the lobes, 

 (P1.38. d.l.) 



The figures of the edge of one transverse plate are repeated in each sue, 

 cessive plate. The animal, as it enlarged its shell, thus leaving behin I it a 

 new chamber, more capacious than the last, so that the edges of the plates 

 never interfere or become entangled. 



Although the pattern on the surflice of this Ammonite is apparently so 



