FLUTED FORM OP RIBS. 257 



First, The entire shell, (PI. 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 apphed in the direction of its longitudinal dia- 

 meter. 



Secondly, besides this general arch-like form, the shell 

 is farther strengthened by the insertion of ribs, or trans- 

 verse arches which give to many of the species their 

 most characteristic feature, and produce in all, that pecu- 

 liar beauty which invariably accompanies the symmetrical 

 repetition of a series of spiral curves. (See PI. 37, Figs. 1 — 

 10.) 



From the disposition of these ribs over the surface of the 

 external shell, there arise mechanical advantages for in- 

 creasing its strength, founded on a principle that is practi- 

 cally applied in works of human art. The principle I allude 

 to, is that by which the strength and stiffness of a thin me- 

 tallic plate are much increased by corrugating, or applying 

 Jlutings 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 corru- 

 gated 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 Am- 

 monites. 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 in- 

 creasing their weight; whilst the intermediate depressed 

 parts between these arches, are suspended and supported 



22* 



