238 HISTORICAL PALAEONTOLOGY. 



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

 shell ; but the principal character which distinguishes Ammon 



Inferior Oolite. 



ites from Goniatitcs 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 



71. Ammonites bifrons. 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 Ammonites range 

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



