MANUAL OF THE MOLLUSC A. 



2. Cybtocerina,* Billings, 1865. 



Type, C. typica, Billings. 



Shell having the general characters of Cyrtoceras, but difiers 

 in the short, thick form, and in the large siphuncle on the 

 dorsal side. 



Distribution, 2 species. Silurian. Canada. 



3. Streptoceras, Billings, 1865. 

 Etymology, streptos, curved, and ceras. 



Shell having the form of Onoceras, but with a trilobed aperture 

 like Phragmoceras. 



Distribution, 2 species. Middle Silurian. Canada. 



LiTUiTES, Breynius.f 



Type, L. lituus, Hisinger. 



Shell discoidal, whorls (2 — 5) close or separate ; last chamber 

 produced in a straight, or nearly straight line, sometimes 

 slightly curved, in a direction contrary to that of the spire ; 

 lateral mo -'-gins of the aperture extended and curved towards 

 the interior of the shell; the aperture contracted thus presents 

 two distinct orifices, the smaller corresponding to the convex or 

 ventral side, the larger to the concave or dorsal side of the shell. 



L. lituus is the only species in which the aperture has been 

 observed. 28 species from the Middle and Upper ? Silurian 

 rocks of Europe and North America, belong here or to allied 

 genera. 



Sub-genus : — Ophidioceeas, Barrande, 1867. 



Synonym, Ophioceras, Barrande, 1865. 



Etymology, ophiodes, shaped like a serpent, and ceras. 



Type, O. Nakholmensis, Kjerulf {Lituites). 



Shell with the produced portion very short or wanting. 



The shells of the Bohemian species are keeled on the convex 

 side. 



Distribution, 7 species. Middle Silurian; Norway (1). Upper 

 Silurian, Bohemia (6). 



LiTUUisrcTJLTJS, Barrande, 1867. 

 Shell as in Lituites, but with a simple aperture. No species 

 have been yet observed. 



Sub-genus : — Discoceras, Barrande, 1867. 

 Etymology, diskos, a quoit, and ceras. 

 Type, D. antiquissimus, Eichwald {Lituites). 

 * See p. 194. t See p. 189. 



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