First, The entire shell, (Pl. 35,) is one continuous arch, coiled spirally around itself in such a manner, that the base of the outer whorls rests upon the crown of the inner whorls, and thus the keel or back is calculated to resist pressure, in the same manner as the shell of a common hen's egg resists great force if applied in the direction of its longitudinal diameter.
Secondly, besides this general arch-like form, the shell is further strengthened by the insertion of ribs, or transverse arches which give to many of the species their most characteristic feature, and produce in all, that peculiar beauty which invariably accompanies the symmetrical repetition of a series of spiral curves. (See Pl. 37, Fig. 1—-10.)
From the disposition of these ribs over the surface of the external shell, there arise mechanical advantages for increasing its strength, founded on a principle that is practically applied in works of human art. The principle I allude to, is that by which the strength and stiffness of a thin metallic plate are much increased by corrugating, or applying flutings to its surface. A common pencil-case, if made of corrugated or fluted metal, is stronger than if the same quantity of metal were disposed in a simple tube. Culinary moulds of tin and copper are in the same way strengthened, by folds or flutings around their margin, or on their convex surfaces. The recent application of thin plates of corrugated iron to the purpose of making self-supporting roofs, in which the corrugations of the iron supply the place, and combine the power of beams and rafters, is founded on the same principle that strengthens the vaulted shells of Ammonites. In all these cases, the ribs or elevated portions, add to the plates of shell, or metal, the strength resulting from the convex form of an arch, without materially increasing their weight; whilst the intermediate depressed parts between these arches, are suspended and supported
