NAUTILID^. 



51 



large reduction to be made in the number of generic forms at 

 present accepted. The following synopsis will exhibit these 

 variations in some of the more important genera. 



Form op Shell. Nautilid^ 



Straight 



Bent or curved . 



Discoidal spire and ( 

 free whorls ( 



IMd. Finally straight 

 or hook-shaped . . . 



Hook-shaped or bent 

 upon itself more than 

 once, whorls free. 



Ibid. Straight portions 

 in contact 



Involute, spiral. . . . 



Orthoceras 



Bactrites 



Gomphoceras . 

 Cyrtoceras. . . . 

 Phragmoceras 

 Ascoceras . . . . 

 Gyroceras 



Ammonitid^. 



With 



undulating 



sutures. 



Baculina. 



Lituites . 



Nautilus — 

 Nothoceras. 



Goniatites. 



Involute; last cham- 

 ber detached, hook- 

 shaped 



Elongated, spiral, 

 whorls in contact. . . 



Spiral, elongated, 

 whorls not in contact 



Spiral, elongated, the 

 last whorl free, pro- 

 duced and recurved. 



Trochoceras 



Clymenia . 



Sutures 

 toothed at 

 the base. 



Sutures 

 complex lobed 

 or foliaceous. 



Baculites. 

 Rhabdoceras 



Toxoceras. 



Ceratites. 



Cryoceras. 



Ancyloceras. 



Hamites. 



Ptychoceras. 

 Ammonites. 



Clydonites, 



Scaphites. 



Turrilites. 

 Cochloceras. 



Helicoceras. 



Heteroceras . 



Family NAUTILID^. 



Orthoceras, Breynius, 1132. 



Etym. — OrthoSj straight, and ceras, a horn. 



Syn. — Actinoceras,Bronn,1835. Orthoceratites,Breyn. Jovel- 

 lania, Bayle, 1878. Cycloceras and Loxoceras, M'Coy, 1844. 



Bistr. — Fossil, 1200 sp. L. Silurian to Triassic; N. America, 

 Australia, Europe, 0. subannulare,BsiYr. (xxix, 79). O.planica- 

 naliculatum, Sandb. (xxviii, 67). 



Shell straight, aperture sometimes contracted. 



Probably the animal was not able to withdraw itself completely 



