TETRABEANOHIATA. 



207 



present accepted. The following synopsis will exhibit these 

 variations in the genera. 



FORM OF SHELL. 



NAUTILID^E. 



Straight Orthoceras . . . 



AMMONITID.E 



With 



undulating 

 sutures. 



Bactrites . . 



Gomphoceras. 



Bent or curved Cyrtoceras. 



j Phragmoceras | 

 Discoidal spire and ( 'Ascoceras 



free whorls ( Gyroceras 



Ibid. Finally straight] 



or hook- shaped . 

 Hook-shaped or bent 



upon itself more than 



once, whorls free.. . . 

 Ibid. Straight portions 



in contact 



Involute, spiral 



Lituites 



Nautilus . . , 

 Nothoceras . 

 Clymenia. . . 



Goniatides. 



Involute ; last cham- 

 her detached, hook- 

 shaped 



Elongated, spiral, 

 whorls in contact . . . 



Spiral, elongated, 

 whorls not in contact jTrochoceras . 



Spiral, elongated, the 

 last whorl free, pro- 

 duced and recurved. 



Sutures 



toothed at 



the base. 



Baculina. 



Sutures 



complex lobed 



or foliaceous. 



Baculites. 

 Rhabdoceras 



iToxoceras. 



Ceratites, 



i Cryoceras. 

 | 

 Ancyloceras. 



j 

 Hamites. 



|Ptychoceras. 

 'Ammonites. 



Clydoiiites. 



Scaphites. 



Turrilites. 

 Cochloceras. 



Helicoceras. 



Heteroceras. 



ORDER II. TETRABRANCHIATA. 



Family 1. NAUTILID.&:. Septa simply curved, concave on the 

 outer face, sutures simple, or undulate or lobed; mouth simple; 

 siphonal opening nearly central. Shell but little sculptured, or 

 smooth. 



Six living- and about (100 fossil species. 



Family 2. AMMONITID/E. Septa convex in their median sec- 

 tion, sutures complex, lobed. ramified or denticulated ; septal 

 tube cylindrical and always directed forwards ; siphuncle cylin- 



