650 



Class CEPHALOPODA. 



Enceplialous mollusks, with locomotive and prehensile organs 

 radiating from the head : dicecious and ametabolian. 



Order TETRABRANCHIATA. 



With four gills : no ink-bladder. Shell external, camerated and 



siphonated. 



Family NautilidcB. Shell discoid or spiral ; dwelling-chamber large ; 

 aperture simple ; sutures simple ; siphon simple, central, or 

 near the concavity of the shell-curve. 



Genera Nautilus, Aiuria, Discites, TemnocheiluSj Cryptoceras, 

 Lituites, Trochoceras, Clymenia. 



Family Orthoceratidce. Shell straight, curved, or discoidal ; dwell- 

 ing-chamber small ; aperture contracted ; siphon central, 

 wide, or complex. 



Genera OrthoceraSy Cameroceras, Actinoceras, Ormoceras, Hu- 

 ronia, Endoceras, Gompfwceras, Onchoceras, Phrag- 

 moceras, Cyrtoceras, Gyroceras, Ascoceras. 



Family AmmonitidcB. Shell discoidal, curved, spiral, or straight ; 

 dwelling-chamber elongated ; aperture guarded by processes 

 and closed by an operculum (called Trigonellates and 

 Aptychus) ; sutures angulated, or lobed and foliated ; 

 siphon peripheral, or at the convexity in the curved 

 shells. 



Genera Goniatites, Ceratites, Amniojiites, Criocerasy Toxoceras, 

 Ancyloceras, Scaphites, Helicoceras^ Turrilites, Ha- 

 iuiteSf Ptychoceras, Baculites. ^ 



