238 



HISTORICAL PALEONTOLOGY. 



the air-chambers is placed on the dorsal or coiivex side of the 

 shell \ but the principal character whieh distinguishes Ammon- 



Yig. 170. — Avunonites H^imphresiamis. Inferior Oolite. 



ites from Go?iiafites and Ceratites is the wonderfully complex 

 manner in which the septa, or partitions between the air-cham- 

 bers, are folded and undulated. To such an extent does this 

 take place, that the edges of the septa, when exposed by the 



Fig. 171. — A nimonites bifro7is. Lias. 



removal of the shell-substance, present in an exaggerated man- 

 ner the appearance exhibited by an elaborately-dressed shirt- 

 frill when viewed edgewise. The species of Am7)io?iites range 

 from the Carboniferous to the Chalk j but they have not been 



