286 MOLLCJSCA. CEPHALOPODA 



Glyphioceras being especially characteristic of the latter. 

 Ammonites appear first in Permian deposits ; throughout 

 the Mesozoic rocks they are extremely abundant, attaining 

 their maximum in the Lias. In the Cretaceous there is 

 a remarkable development of more or less completely un- 

 coiled Ammonoids, e.g., Hamites, Macroscaphites (fig. 115), 

 Bacidites, Crioceras, and Scaphites ; and there is evidence 

 showing that, in most cases, these 'genera' include species 

 which have descended from more than one genus of am- 

 monites. The abrupt disappearance of the Ammonoidea 

 at the end of the Cretaceous period is remarkable. 



Dibranchia. The Dibranchia are more numerous and 

 more varied in existing seas than they were at any former 

 period. Some forms are pelagic, others abyssal, but the 

 larger number are found in littoral regions and are dis- 

 tributed in provinces similar to those of other molluscs 

 (p. 251); typical littoral genera are Octopus, Sepia, and 

 Loligo. 



The Dibranchia are unknown in the Palaeozoic forma- 

 tions, the earliest examples {Aulacoceras, Phragmoteuthis) 

 appearing in the Trias. Belemnites is the chief form in 

 the Jurassic and Cretaceous, and is especially abundant 

 in the clayey beds. Geoteuthis occurs in the Lias; and 

 Belemnoteuthis, Plesioteuthis, etc. in the Upper Jurassic. 

 Belemnitella is limited to the Upper Chalk. Dibranchs 

 are relatively rare in the Eocene and Miocene ; Belemnites 

 is absent, but is represented by Belosepia and Spiruli?*ostra. 



The principal genera of Cephalopoda are : 



Cambrian. Volborthella in the Lower Cambrian ; Orthoceras 

 and Cyrtoceras in the Tremadoc Beds. 



Ordovician. Orthoceras, Cyrtoceras, Endoceras, Piloceras, Cono- 

 ceras. 



