MANUAL OF THE MOLLUSCA, 
2. CYRTOCERINA,* Billings, 1865. 
Type, C. typica, Billings. 
Shell haying the general characters of Cyrtoceras, but difters 
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. 
Litvurres, Breynius.+ 
Type, Lu. 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 margins of the aperture extended and curyed 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 dorsai 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 Kurope and North America, belong here or to allied 
genera. 
Sub-genus :—OPHIDIOCERAS, Barrande, 1867. 
Synonym, Ophioceras, Barrande, 1865. 
Etymology, ophiodes, shaped like a serpent, and ceras. 
Type, O. Nakholmensis, Kjerulf (Litwites). 
Shell with the produced portion very short or wanting. 
The shells of the Bohemian species are keeled on the conyex 
side. 
Distribution, 7 species. Middle Silurian; Norway (1). Upper 
Silurian, Bohemia (6). 
LITUUNCULUS, Barrande, 1867. 
Shell asin Litwites, but with a simple aperture. No species 
have been yet observed. 
Sub-genus :—DIscocEras, Barrande, 1867. 
Kitymology, diskos, a quoit, and ceras. 
Type, D. antiquissimus, Hichwald (Lituites). 
P * See p. 194. 7 See p. 189. 
