OF CREATION. 143 



to combine the greatest resisting power with the 

 smallest weight of material. Additional strength is 

 also gained by the bosses or elevations of part of 

 the ribbed surface into dome-shaped tubercles ; for 

 these, like the vaultings in architecture, give strength 

 to the surface to be supported, and are therefore 

 usually placed at those parts of the external shell 

 beneath which there is no immediate support from 

 the internal walls which separate the chambers from 

 one another. 



It is, however, chiefly in the arrange- pig. 55 

 ment and construction of these walls of s 



separation (fig. 55), that mechanical con- 

 trivance seems carried to its height in the 

 shell of the ammonite ; but here, also, 

 the contrivance is exceedingly simple, 

 and merely consists in causing the extre- SEPTUM 



P ., ii i ,1 , or Chamber-wall 



mities of the walls, where they meet O f an 

 the shell, to deviate into a variety of AMMONITE. 

 ramifications and undulating lines. This is singularly 

 shewn in the progressive change by which the Nauti- 

 lus (fig. 53, p. 139), where these lines of intersection are 

 nearly straight, is succeeded by the Clymenia, where 

 they are decidedly curved ; then by the Goniatite (fig. 

 40. p. 96), where they become angular ; then again by 

 the Ceratite (fig. 45, p. 119), where they are rounded 

 and exhibit a tendency to undulations; and lastly by 

 the Ammonite, in many species of which * the sinuous 

 windings of these sutures, at their union with the 

 external shell, are singularly complicated and beau- 



* See figs. 102, 104, where are represented closely allied forms, in 

 which the nature of the intersection does not diifer from that observed in 

 the most typical ammonites. 



