650 



Class CEPHALOPODA. 



Encephalous mollusks, with locomotive and prehensile organs 

 radiating from the head : dioecious and ametabolian. 



Order TETRABRANCHIATA. 



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



siphonated. 



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

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

 near the concavity of the shell-curve. 



Genera Nautilus, Aiitria, Discites, Temnocheilus, Cryptoceras, 

 Lituites, Trochoceras, Clymenia, 



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

 ing-chamber small ; aperture contracted ; siphon central, 

 wide, or complex. 



Genera Orthoceras, Cameroceras, Actinoceras, Ormoceras, Hu- 

 ronia, Endoceras, Gomphoceras, Onchoceras, Phrag- 

 moceras, Cyrtoceras, Gyroceras, Ascoceras. 



Family Ammonitidce. 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 j 

 siphon peripheral, or at the convexity in the curved 

 shells. 



Genera Goniatites, Ceratites, Ammonites, Crioceras, Toxoceras, 

 Ancyloceras, Scaphites, Helicoceras, Turrilites, Ha- 

 mites, Ptychoceras, Baculites. 



