182 MOLLUSCA. CEPHALOPODA. 



in the Trias Ceratites is characteristic ; in the Cretaceous 

 we get a remarkable development of new genera, Turri- 

 lites, Hamites, Scaphites, Crioceras, and Baculites, all of 

 these except the first, being evolute forms. At the end of 

 the Cretaceous, the Ammonoids become extinct in a 

 remarkably sudden manner, except in California, where 

 some Ammonites have been found in the lower Tertiary 

 beds. 



The Dibranchiata are more numerous and more varied 

 in existing seas, than they were at any former period. 

 They are most abundant in littoral regions and are distri- 

 buted in provinces similar to the other mollusks ; typical 

 littoral genera are Octopus, Sepia and Loligo. Some 

 forms are pelagic, others abyssal. The Dibranchiata are 

 unknown in the Palaeozoic, the earliest examples appearing 

 in the Trias. Throughout the Mesozoic, Bekmnites is the 

 chief form, and is very abundant especially in the clayey 

 beds. In the Jurassic, Geoteuthis and Belemnoteuthis also 

 occur; and in the Upper Cretaceous, Belemnitella. In 

 the Tertiary, Belosepia and Beloptera, are the principal 

 genera. 



