GENEEA OF AMMONITID^. 



79 



modes of subdivision have been adopted by different pale- 

 ontologists. In one system, the principal character relied 



Fig. 468. — Ammonites inargarUatus. Lias. 



Fis;. 469. — Ammonites tisulcatus. Lias. 



upon is the form of the dorsal side or convex margin of the 

 shell, and in accordance with this the genus may be divided 

 into the following groups (Pictet) : — 



SECTioisr A. Back tvith an entire heel. 



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 Ehotomagenses . . Cretaceous. — Ex. A. crenatus. 



Sectiox C. Back compressed and sharp. 



6. Glypeiformi (or Disci) . . . Oolites. — Ex. A. discus. 



7. Dentati . 



8. Gemmati 



9. Flexuosi . . 



10. Compressi . 



11. Armati . . 



12. Angulicostati 



13. Caioricorni . 



14. Heterophylli 



15. Ligati . . 



16. Planulati . 



Section D. Bctck channelled. 



Oolitic and Cretaceous. — Ex. A. Jason. 

 Trias. — Ex. A. Aon. 



Section E. Back squared. 



Cretaceous. — Ex. A. radiatus. 

 Cretaceous. — Ex. A. Beaumontianus. 

 Oolitic. — Ex. A. jjerarmatus. 

 Oolitic and Cretaceous. — Ex. A. Milletianus. 



Section F, Back round. 



Lias. — Ex. A. planicostatus. 

 Oolitic. — Ex. A. heteroptyllus. 

 Cretaceous. — Ex. A. Mayorianus. 

 Oolitic. — Ex. A. annulatus. 



