THE JURASSIC PERIOD. 245 



with its many sub-genera and its hundreds of recorded species. 

 The shell in Ammonites is in the form of a flat spiral, all the 

 coils of which are in contact (figs. 170 and 171). The inner- 



rig. 170. Ammonites Humphresianus . Inferior Oolite. 



most whorls of the shell are more or less concealed; and the 

 body-chamber is elongated and narrow, rather than expanded 

 towards the mouth. The tube or siphuncle which runs through 

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



Fig. 171. Ammonites bifrona. Lias. 



shell; but the principal character which distinguishes Ammon- 

 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 



