/O 



THE GEOLOGIST. 



slight tubercules which shew themselves on the arch of the 

 umbilicus. These, at first scarcely visible, gradually elevate 

 themselves and remain single for a shorter or longer period^ 

 according to the species ; it is generally at this age that the 

 keel commences to shew itself. In some ammonites which 

 have no other external ornaments_, as A. LalUeriensis, {A. 

 inflatus, Reineck), A. peramplus, for example, this appears 

 to be the complete state, but the greater number undergo 

 still further changes of form {A. interruptus^ Martinii, as- 

 perrimus, Dejiarius^ auritus, Lewesiensis, Camatteanus, In- 

 signis, Deshayesi). Sometimes when there are no tubercules 

 it is the ribs or striae which appear at this period of the 

 growth of ammonites. 



Third modification which I consider as the last period of 

 growth {derniere periode d'accroissement). Whilst the tu- 

 bercules and the ribs on the arch of the umbilicus become 

 more strongly marked and raised, we observe sooner or 

 later, according to the species, on the back or sides, the 

 simple and interrupted ribs, nodosities, and tubercules which 

 are to receive the modifications proper to each species, 

 gradually appear. If an ammonite be only destined to pre- 

 sent a single range of tubercules on the middle or sides of 

 the back, it is complete ; but if it bears many, as we remark 

 in A. mammillaris and Lyelli, these succeed each other from 

 the exterior to the interior to the full number fixed by nature. 

 When the ammonites commence to join the ornaments of the 

 back to those of the arch of the umbilicus, all develope them- 

 selves during a certain period and then the species are 

 covered with all the richness of ornament they can attain. 

 This state, which is the most durable, since it occupies gene- 

 rally three fourths of the existence and growth of the ammo- 

 nites, may be regarded as the adult state. It is this period 

 which we must choose to describe, a species with its maxi- 

 mum of specific characters {A, Leopoldmus, interrupts, dis- 



