The bosses thus often introduced at the origin, division, and termination of the ribs, resemble those applied by architects to the intersections of the ribs in Gothic roofs, and are much more efficient in producing strength.[1] These tubercles have the effect of little vaults or domes; and they are usually placed at those parts of the external shell, beneath which there is no immediate support from the internal transverse plates (see Pl. 37, Fig. S. Pl. 42, Fig. 3. c. d. e. and Pl. 40, Fig. 5.)[2]
- ↑ The ribs and bosses in vaulted roofs project beneath the under surface of the arch; in the shells of Ammonites, they are raised above the convex surface.
- ↑ In Pl. 37, Fig. 9 (A. varians,) the strength of the ribs and proportions of the tubercles are variable, but the general character exhibits a triple series of large tubercles, rising from the surface of the transverse ribs. Each of these ribs commences with a small tubercle near the inner margin of the shell. At a short distance outwards is a second and larger tubercle, from which the rib bifurcates, and terminates in a third tubercle, raised at the extremity of each fork upon the dorsal margin.
Many species of Ammonites have also a dorsal ridge or keel, (Pl. 37, Figs. 1. 2. 6.) passage along the back of the shell, immediately over the siphuncle, and apparently answering, in some cases, the further purpose of a cut-water, and keel (Pl. 37, Figs. 1, 2.) In certain species, e. g. in the A. lautus (Pl. 37, Fig. 7, a. c.) there is a double keel, produced by a deep depression along the dorsal margin; and the keels are fortified by a line of tubercles placed at the extremity of the transverse ribs. In the A. varians (Pl. 37, 9. a. b. c.) which has a triple keel, the two external ones are fortified by tubercles, as in Fig. 7, and the central keel is a simple convex arch.
Pl. 37, Fig. 8, offers an example of domes, or bosses, compensating the weakness that, without them, would exist in the A. catena, from the minuteness of its ribs, and the flatness of the sides of the shell. These flat parts are all supported by an abundant distribution of the edges of the transverse plates directly beneath them, whilst those parts which are elevated into bosses, being sufficiently strong, are but slightly provided with any other support. The back of this shell also, being nearly flat, (Pl. 37, Fig. 8. b. c.) is strongly supported by ramifications of the transverse plates.
In Pl. 37, Fig. 6, which has a triple keel, (that in the centre passing over the siphuncle,) this triple elevation affords compensation for the weakness that would otherwise arise from the great breadth and flatness
