238 HISTORICAL PALASONTOLOGY. 
the air-chambers is placed on the dorsal or conzex side of the 
shell; but the principal character which distinguishes Ammon- 
Fig. 170.—A mmonites Humphrestanus. Inferior Oolite. 
ites from Gonziatites and Ceratites is the wonderfully complex 
manner in which the sea, 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 ; but they have not been 
