HISTORICAL PALEONTOLOGY. 



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

 shell; but the principal character which distinguishes Amman- 



Fig. 170. Ammonites Humphresiamis. Inferior Oolite. 



ites from Goniatites 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. 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 j but they have not been 



