Belongs within: Orthoceratidae.
Orthoceras is a Middle Ordovician genus of large orthocerid cephalopods. The genus has historically been ill-defined, initially being applied to all straight-shelled cephalopods.
Characters (Sweet 1964): Slightly expanding, almost cylindrical orthocones or faintly exogastric cyrtocones; siphuncle empty, cylindrical, suborthochoanitic, subcentral to slightly ventral. Body chamber transversely constricted at mid-length and marked by single longitudinal dorsal and two longitudinal ventral depressions. Surface ornamented by network of fine lirae; internal surface faintly striated longitudinally, prominent conchal furrow on venter; muscle attachment dorsomyarian.
<==Orthoceras Bruguière 1789 [=Ortaoceras (l. c.), Orthoceros Brünnich 1771 (nom. rej. prop.)]S64
|--*O. regulare (Schlotheim 1820) [=Orthoceratites regularis, *Orthoceros regularis]S64
|--O. aciculoides Clarke 1885P68
|--O. alienoides Foerste 1928P68
|--O. angulatumS64
|--O. asmodeus Clarke 1885P68
|--O. bicinctumG31
|--O. byronense Foerste 1928P68
|--O. caereesienseM87
|--O. caneyanum Girty 1909P68
|--O. carltonense Whitfield 1878P68
|--O. choctawense Girty 1909P68
|--O. clarki Cleland 1911P68
|--O. clarksvillense Foerste 1924P68
|--O. crebriliratum Girty 1909P68
|--O. erraticum Foerste 1893P68
|--O. eurekensis Walcott 1884P68
|--O. filosum Clarke 1885P68
|--O. franklinense Miller 1894P68
|--O. furtivumW77
|--O. guadelupense Girty 1908P68
|--O. imbricatumP68
|--O. inceptum Foerste 1885P68
| |--O. i. inceptumP68
| `--O. i. acceleratum Foerste 1893P68
|--O. indianum Firty 1909P68
|--O. junceumP68
|--O. kingii Meek 1877P68
|--O. kodinaeG31
|--O. lambtonense Whiteaves 1898P68
|--O. lesueuri Clarke 1897P68
|--O. liaotungense Endo 1932P68
|--O. ludenseH87
|--O. magnichinense Endo 1932P68
|--O. mephisto Clarke 1885P68
|--O. moodiense Foerste 1928P68
|--O. multicameratumP68
|--O. nilssoni (Boll 1857)K93
|--O. obliqueannulatumG31
|--O. olorusP68
| |--O. o. olorusP68
| `--O. o. baffinense Schuchert 1900P68
|--O. ontario Clarke 1885P68
|--O. orientaleG31
|--O. parlinense Williams & Breger 1916P68
|--O. porteri Schuchert 1900P68
|--O. princiana Williams & Breger 1916P68
|--O. pulchrumW77
|--O. randolphensisP68
|--O. rectiseptatum Endo 1932P68
|--O. rogersensis Foerste 1914P68
|--O. rushensisP68
|--O. scalariformis Schuchert 1900P68
|--O. shatzeri Foerste 1928P68
|--O. toyamai Endo 1932P68
|--O. tuba Girty 1911P68
|--O. tyronensis Foerste 1912P68
|--O. undulostriatumW77
|--O. wahlenbergi (Boll 1857)K93
|--O. wapanuckense Girty 1909P68
|--O. whitfieldi Foerste 1928P68
`--O. wilmingtonense Foerste 1928P68
*Type species of generic name indicated
References
[G31] Grabau, A. W. 1931. The Permian of Mongolia: A report on the Permian fauna of the Jisu Honguer limestone of Mongolia and its relations to the Permian of other parts of the world. American Museum of Natural History: New York.
[H87] Holland, C. H. 1987. The nautiloid cephalopods: a strange success story. Journal of the Geological Society 144 (1): 1–15.
[K93] King, A. H. 1993. Mollusca: Cephalopoda (Nautiloidea). In: Benton, M. J. (ed.) The Fossil Record 2 pp. 169–188. Chapman & Hall: London.
[M87] Mitchell, J. 1887. Notes on the geology of Bowning, N.S.W. Proceedings of the Linnean Society of New South Wales, series 2, 1 (4): 1193–1204.
[P68] Purnell, L. R. 1968. Catalog of the type specimens of invertebrate fossils. Part I: Paleozoic Cephalopoda. United States National Museum Bulletin 262: 1–198.
[S64] Sweet, W. C. 1964. Nautiloidea—Orthocerida. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt K. Mollusca 3. Cephalopoda—General Features—Endoceratoidea—Actinoceratoidea—Nautiloidea—Bactritoidea pp. K216–K261. The Geological Society of America and the University of Kansas Press.
[W77] White, C. A. 1877. Report upon the invertebrate fossils collected in portions of Nevada, Utah, Colorado, New Mexico, and Arizona, by parties of the expeditions of 1871, 1872, 1873, and 1874. U.S. Geographical Surveys West of the One Hundredth Meridian 4 (1): 1–219, pls 1–21.