252 PALEONTOLOGY 



occasioned by age, sex, and diseased .conditions, has iden- 

 tified 530 species of this genus. They are represented by 

 numerous species in the triasic, and are very abundant in 

 all the stages of the oolitic and cretaceous periods. In each 

 of the eighteen geological stages in which ammonites are 

 found, certain groups of specific forms, distinguished by Yon 

 Buch by appropriate names, are found to characterise certain 

 beds ; thus, the groups known as arietes and falcifera are 

 special to the lias. Amalthei, ornati, capricorni, are oolitic. 

 Flexuosi is neocomian; cristati, tuberculati, clypeiformi, 

 pulchelli, rhotomagenses, and dentati, are cretaceous. 



FIG. 170. Ammonites Jason. Oxford clay. 



The ancyloceras (fig. 173) and JieMcoceras commenced their 

 existence in the seas of the great oolite, and extended into 

 the upper stages of the chalk. The scaphites, faculties, Jia- 

 mites, turrilites, crioceras, ptycJioceras, and heteroceras, are 

 generic forms special to the cretaceous period. 



The ACETABULIFEEA, D' Orb., (Dibranchiata, Owen.) 

 form the most highly organised group of this class. The 

 arms are provided with acetabula, or sucking discs, for adhe- 



