54 NAUTILID^. 



Shell straight, conical ; a small sutural lobe corresponds to 

 the marginal siphon. 



PiLOCERAs, Salter, 1859. 



Etym. — Pilos, a cap, and ceras, a horn. 



Distr. — Fossil, 3 species. L. Silurian ; Canada, Scotland 

 (Ideal section, xxx, 9'!). 



Shell broad, conical, subcylindrical or compressed, slightly 

 curved. The siphuncle and septa represented by a series of 

 conical septa, concave to a central point. Closely related to 

 Cyrtoceras. 



Cyrtoceras, Goldfuss, 1832. 



JEtym. — Ctcrtos, curved, ceras, a horn. 



Sy7i. — Aploceras, d'Orb., 1850. Campulites, Desh. (part), 1832. 

 Campyloceras and Trigonoceras, M'Coy, 1844. 



Diatr. — Fossil, 350 species. L. Silurian to Carb. ; JS". and S. 

 America, Europe. G. acuticostatum, Sandb. (xxx, 98). 



Shell curved ; siphuncle small, subcentral. 



Seems to differ but little from Orthoceras. 



ONCOCERAS, Hall, 184*7. (0?rcos, a protuberance, ceras,a horn.) 

 Anterior half of the shell inflated, aperture more or less stran- 

 gulated. This may possibly = Phragmoceras, Brod. Distr. — 

 3 sp. Silurian; New York. 0. constrictum. Hall (xxx, 997). 



OYRTOCERiNA, Billings, 1865. Shell short and thick, with a 

 large siphuncle, placed externally. Distr. — 2 species. Silurian ; 

 Canada. 



STREPTOCERAS, Billiugs, 1865. Shell like Oncoceras, but the 

 aperture trilobed. 2 species. Middle Silurian ; Canada. 



GoMPHOCERAS, J. Sowb., 1839. 



Etym. — Gomphos, a club, and ceras, a horn. 



Syn. — Apioceras, Fischer, 1844. Poterioceras, M'Coy, 1844. 

 Mesoceras, Barrande, 1877. Bolboceras, Fischer, 1844. Neli- 

 meuia, Casteln., 1843. 



Distr. — 100 sp. L. Silurian to Carb. ; Europe, N. America. 

 G. pyriforme., Murchison (xxx, 100). Silurian ; England. G. 

 Bohemicum, Barr. (xxx, 1). Aperture. 



Shell fusiform or bottle-shaped, straight, swollen anteriorly ; 

 aperture contracted in the middle ; siphuncle subcentral ; septa 

 simple, concave. 



Sycoceras, Pictet, 1854. 



Distr. — Silurian, Devonian. S. ortJiogaster^ Sandb. (xxx, 21). 



Shell oval or bottle-shaped, straight ; septa simple ; siphuncle 

 marginal. 



The position of the siphon varies so much in this group, that 

 it is an insufficient character to distinguish the genus from 

 Gomphoceras. 



