VAULTED DOMES AND BOSSES. 259 



The bosses thus often introduced at the origin, division, and 

 termination of the ribs, are placed hke those apphed by archi- 

 tects to the intersections of the ribs in Gothic roofs, and are 

 much more efficient in producing strength.* These tuber- 

 cles have the eftect 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 trans- 

 verse plates (see PI. 37, Fig. 8. PI. 42. Fig. 3. c. d. e. and 

 PI. 40. Fig. 5.)t 



* The ribs and bosses in vaulted roofs project beneath the under surface 

 of the arch; in the sliclls of Ammonites, they are raised above the convex 

 surface. 



T In PI. 37, Fig. 9. (A. varians,) the strength of the ribs and proportions 

 of the tubercles arc variable, but the general character exhibits a triple se- 

 ries of-large tubercles, rising from the surface of the transverse ribs. Each 

 of lliese ribs commences with a small tubercle near the inner margin of the 

 shell. At a short distance outwards is a second and l.irger tubercle, from 

 which the rib bifurcates, and terminates in a third tubercle, raised at the 

 extremity of each fork upon tlie dorsal margin. 



Many species of Ammonites have also a dorsal ridge or keel, (PI. 37, 

 Figs. 1. 2. 6.) passing along the back of the shell, immediately over the 

 siphuncle, and apparently answering, in some cases, the farther purpose 

 of a cut-water, and keel (PI. 37, Figs. 1, 2.) In certain species, e. g. in 

 the A. lautus (PI. 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 (PI. 37, 9. a. b. c.) which lias a triple keel, the two external ones 

 are fortitied by tubercles, as in Fig. 7, and the central keel is a simple con- 

 vex arch. 



PI. 37, Fig. 8. offers an example of domes, or bosses, compensating 

 tlie 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 

 fiat parts are all supported by an abundant distribution of tiie edges of 

 the transverse plates directly beneath them, whilst those parts which arc 

 elevated into bosses, being sufficiently strong, are but slightly provided wilh 

 any other support. The back of this shell also, being nearly flat, (Pi. 

 37, Fig. 8. b. c.) is strongly supported by ramifications of the transverse 

 plates. 



In PI. 37. Fig. G, which has a triple keel, (that in the centre passing over 

 the siphuncle,) this triple elevation afi'ords compensation for the weakness 

 that would otherwise arise from the great breadth and flatness of the dorsal 



