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Ophidioceras
''Ophidioceras'' is an alternate name for the Tarphycerid genus ''Ophioceras'', which is known from the Upper Silurian of Europe and North America.〔W.M Furnish and Brian F Glenister 1964. Nautiloidea-Tarphyhceria. Treatise on Invertebrate Paleontology Part K. Geological Soc of America and Univ Kansas Press.〕
''Ophioceras'', as this genus is more properly known, has a narrow, evolutly wound, closely coiled shell that diverges in the adult stage. The shell is more or less circular in cross section, strongly annulate with close spaced, rounded, transverse ribs. The venter is bicarinate, with keels along either side separated by a flattened surface. The siphuncle is tubular and thin walled, and may be slightely ventral or dorsal of the center. The adult aperture has deep hypononic and ocular sinuses, not too dissimilar from the Ordovician ''Lituites'', to which this genus is possibly allied. 〔〔Charles Hepworth Holland 2010. Coiled nautiloid cephalopods from the British Silurian. Proceedings of the Geologists’ Association 121 (2010) 13–23 ()〕
Some 12 species have been described which have been reduced to two, ''Ophioceras rudens'' Barrande, 1865 and ''O. simplex'' Barrande, 1865 with two subspecies, ''O. simplex simplex'' and ''O. s. proximum''. Minor evolutionary changes can be observed within both species, which In spite of their widespread distribution do not demonstrate any major geographical variations.〔Stridsberg, S. & Turek, V., 1997: A revision of the Silurian nautiloid genus Ophioceras Barrande. GFF, Vol. 119 (Pt. 1, March), pp. 21–36. Stockholm. ISSN 1103–5897; Abstract.()〕
==References==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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