342 VAULTED DOMES AND BOSSES. 
cles, or bosses, thus superadding the strength of 
a dome to that of the simple arch, at each point 
where these bosses are inserted.* 
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 effi- 
cient in producing strength.! These tubercules 
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 sup- 
port from the internal transverse plates (see 
PI. 37, Fig. 8. PI. 42, Fig. 3. c. d. e. and PI. 40, 
Fig. 5.)| 
* These places are usually either at the point of bifurcation, 
as in PL 37, Figs. 2, 7, 9, 10, or at the point of trifurcation, as 
in Fig. 3. 
f 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. 
X In PI. 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, 
(PI. 37, Figs. 1. 2. 6.) passing along the back of the shell, imme- 
diately over the siphuncle, and apparently answering, in some 
cases, the further purpose of a cut-water, and keel (PI. 37, Figs. 
1, 2.). In certain species, e. g. in the A. lautus (PI. 37, 
