T K T R A B RAN C H I A T A . 



present accepted. The following synopsis will exhibit these 

 variations in the genera. 



FORM OF SHELL. NAUTILID.E. 



AMMONITID.E . 



With 



undulating 

 sutures. 



Straight Orthoceras . . . Bactrites . . 



Gomphoceras . 



Bent or curved Cyrtoceras 



j Phragmoceras 



Discoidal spire and ( iAscoceras ; 



free whorls | Gyroceras 



Ibid. Finally straight; 



or hook-shaped jLituites 



Hook-shaped or bent 

 upon itself more than 



once, whorls free.. . . 



Ibid. Straight portions 



in contact } 



Involute, spiral Nautilus. . . 



Nothoceras. 

 Clymenia. . . 



Goniatides. 



Involute ; last chain-! 

 her detached, hook- 

 shaped i 



Elongated, spiral, 

 whorls in contact . . . 



Spiral, elongated, 

 whorls not in contact jTrochocer a ,- 



Spiral, elongated, the' 

 last whorl free, pro- 

 duced and recurved. 



Sutures 



Sutures 



toothed at 'complex lobed 

 the base. I or foliaceous. 



Baculina. JBaculites. 



. iRhabdoceras 



. IToxoceras. 



jCryoceras. 

 Ancyloceras. 



Hamites. 



Ptychocei 

 Ceratites. ! Ammonites. 



jPtychoceras. 



Kif.AH I A mi 



Clydonites. 



,Scaphites. 



Turrilites. 

 Cochloceras. 



Helicoceras. 



Heteroceras. 



ORDER II. TETRABRANCHIATA. 



Family 1. NAUTILID^E. Septa simply curved, concave on the 

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

 siplxmal opening nearly central. Shell but little sculptured, or 

 smooth. 



Six living ;ind about fiOO 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- 



