258 SUBDIVISION OF RIBS. 



by the tenacity and strength of the material. (Sec PI. 37, 

 Figs. 1—10.*) 



The general principle of dividing and subdividing the ribs, 

 in order to multiply supports as the vault enlarges, is con- 

 ducted nearly on the same plan, and for the same purpose, 

 as the divisions and subdivisions of the ribs beneath the groin 

 irorli, in the flat vaulted roofs of the florid Gothic Architec- 

 ture. 



Another source of strength is introduced in many species 

 of Ammonites by the elevation of parts of the ribs into little 

 dome-shaped tubercles, or bosses, thus superadding the 

 strength of a dome to that of the simple arch, at each point 

 where these tubercles are inserted.^ 



* The figures engraved at PI. 37, afford examples of various contrivances 

 to give strength and beauty to the external shell. The first and simplest 

 mode, is that represented in PI. 35 and PI. 37, V'lg. 1 and 6. Here each 

 rib is single, and extends over the wliole surface, becoming graduall)' wider, 

 as the space enlarges 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 tlie inner margin, soon bifurcate into two ribs that 

 extend outwards, and terminate upon the dorsal keel. 



In tlie third case, (Pi. 27. Fig. 4.) the ribs originate simply, and bifur- 

 cating as the shell enlarges, extend this bifurcation entirely around its circu- 

 lar back. Between each pair of bifurcated ribs, a third or auxiliary sliort rib 

 is interposed, to fill up the enlarged space on the dorsal portion where the 

 shell is broadest. 



In the fourth modification, (Pi. 37, Fig. 3,) the ribs originating singly on 

 the internal margin, soon become trifurcatc, and expand outwards, around 

 tlie circular back of the shell. A perfect mouth of this shell is represented 

 attl. 37,Fig. 3.d. 



A fifth case is that (PI. 37, Fig. 5,) in which the simple rib becomes tri- 

 furcatc as tiie space enlarges, and one or more aiixiliary sliort ribs are also 

 interposed, between each trlfurcation. Tiiese 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 tliose spaces whicli the ex- 

 pansion of the shell towards its outer circumference, would have rendered 

 weak without the aid of some such compensation. 



j- These places are usually either at the point of bifiu-cation, as in PI. 37, 

 I'lgs. 2; 7, 9, 10, or at tiie point of trifurcation, as in Fig. 3. 



