SUB-DIVISION OF RIBS. 341 
The general principle of dividing and sub- 
dividing the ribs, in order to multiply supports as 
the vault enlarges, is conducted nearly on the 
same plan, and for the same purpose, as the 
divisions and subdivisions of the ribs beneath 
the groin tcork, in the flat vaulted roofs of the 
florid Gothic Architecture. 
Another source of strength is introduced in 
many species of Ammonites by the elevation of 
parts of the ribs into little dome-shaped tuher- 
PL 37, Fig". 1 and 6. Here each rib is single, and extends over 
the whole surface, becoming gradually wider, as the space en- 
larges towards the outer margin, or back of the shell. 
The next variation is that represented (PI. 37, Figs. 2, 7, 9,^ 
where the ribs, originating singly on the inner margin, soon bi- 
furcate into two ribs that extend outwards, and terminate upon 
the dorsal keel. 
In the third case, (PI. 37. Fig. 4), the ribs, originate simply, 
and bifurcating as the shell enlarges, extend this bifurcation 
entirely around its circular back. Between each pair of bifur- 
cated ribs, a third or auxiliary short rib is interposed, to fill up 
the enlarged space on the dorsal portion where the shell is 
broadest. 
In a fourth modification, (PI. 37, Fig. 3), the ribs, originating 
singly on the internal margin, soon become trifurcate, and 
expand outwards, around the circular back of the shell. A 
perfect mouth of this shell is represented at PI. 37, Fig. 3, d. 
A fifth case is that (PI. 37, Fig. 5,) in which the simple rib 
becomes trifurcate as the space enlarges, and one or more 
auxiliary short ribs are also interposed, between each trifurcation. 
These subdivisions are not always maintained with numerical 
fidelity through every individual of the same species, nor over the 
whole surface of the same shell ; their use, however, is always 
the same, viz. to cover and strengthen those spaces which the 
expansion of the shell towards its outer circumference, would 
have rendered weak without the aid of some such compensation' 
