2QO 



MOLLUSCA. 



Chalk, and are thus exclusively confined to the Secondary period. 

 Within these limits, each rock-group is characterised by par- 

 ticular species, the number of individuals being often very great, 

 and the size which is sometimes attained being nothing short of 



Fig. 261. Ammonites bifrons. Lias. 



gigantic. In the Lias particular species of Ammonites succeed 

 one another regularly, each having its own definite horizon, 

 which it does not transgress. It is thus possible to distinguish 

 a certain number of zones, each characterised by a particular 

 Ammonite. Some of these zones are very persistent and ex- 

 tend over very wide areas, thus affording valuable aid to the 

 geologist in his determination of rocks. It is to be remem- 

 bered, however, that there are other species which are not thus 

 restricted in their vertical range, even in the same formations 

 in which definite zones occur. 



The numerous species of Ammonites are divided into groups 

 as follows (Pictet) : 



SECTION A. Back with an entire keel. 



1. Arietes . . . Lower Oolites. Ex. A. bisulcatus. 



2. Falciferi . . Lower Oolites. Ex. A. serpentinus. 



3. Cristati . . . Cretaceous. Ex. A. inflatus. 



SECTION B. Back crenated or tuberculated. 



4. Amalthei . . Oolites. Ex. A. cordatus. 



5. Pulchelli (or Rhotomagenses) . . Cretaceous. Ex. A. crenatus. 



SECTION C. Back compressed and sharp. 



6. Clypeiformi (or Disci) . . . Oolites. Ex. A. discus. 



