78 CEPHALOPODA. 



ular species, the number of individuals being often very 

 great, and the size which is sometimes attained being noth- 

 ing short of gigantic. In the Jurassic rocks particular 

 species of Ammonites are associated with particular groups 



Y'v^. Jilj. — Aiiiiiionitesbifrons. Lias. 



of fossils at definite horizons ; and it is thus possible to 

 distinguish a certain number of zones, each especially char- 

 acterised by a particular Ammonite. Some of these zones 

 are very persistent and extend over very wide areas, thus 

 affording valuable aid to the geologist in his determination 

 of rocks. It is to be remembered, however, that there are 



Fig. 4(35. — Ammonite.'; Jason. Jurassic. 



Fig. 407. — Ammonites conltttus. Jurassic. 



other species which are not thus restricted in their vertical 

 range, even in the same formations in which definite zones 

 occur. 



The number of species included under the general name 

 Ammonites is so large, that it is absolutely necessary to 

 break up the genus in some manner or another, and various 



