Here again we find the inventions of art anticipated in 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 beneath 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 Nautili; 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 Nautili.
This support is effected by causing the edges of the transverse plates to deviate from a simple curve, into a variety of attenuated ramifications and undulating sutures. (See Pl. 38. and Pl. 37, Figs. 6, 8.) Nothing can be more beautiful than the sinuous windings of these sutures in many species, 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.[1]
- ↑ The A. Heterophyllus, (Pl. 38,) is so called from the apparent occurrence of two different forms of foliage; its laws of denotation are the same as in other Ammonites, but the ascending secondary saddles (Pl. 38; S. S.) which, in all Ammonites are round, are in this species longer than ordinary, and catch attention more than the descending points of the lobes, (Pl. 38. d. 1.)
The figures of the edge of one transverse plate are repeated in each successive plate. The animal, as it enlarged its shell, thus leaving behind 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 surface of this Ammonite is apparently so
