''Ophioceras'' is a genus of closely coiled tarphycerid nautiloid cephalopods, the sole representatives of the family Ophidioceratidae, characterized by an

evolute In the differential geometry of curves, the evolute of a curve is the locus (mathematics), locus of all its Center of curvature, centers of curvature. That is to say that when the center of curvature of each point on a curve is drawn, the result ...

shell with narrow, subrounded, annulated whorls and a subcentral

siphuncle The siphuncle is a strand of biological tissue, tissue passing longitudinally through the mollusc shell, shell of a cephalopod mollusc. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the li ...

composed of thin connecting rings that show no evidence of layering. The mature body chamber is strongly divergent and is the longest proportionally of any tarphycerid. The aperture has a deep hyponomic sinus and ocular sinuses, and so resembles some lituitids. ''Ophioceras'' seem to have its origin in the

Trocholitidae The Trocholitidae are Tarphycerida with Whorl (botany), whorls in close contact as with the Tarphyceratidae, but in which the siphuncle, similar in structure, becomes dorsal. The Trocholitidae are derived from the Tarphyceratidae, perhaps from di ...

, possibly in '' Graftonoceras''. 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'' has a narrow, evolutely 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 slightly ventral or dorsal of the center. The adult aperture has deep hyponomic and ocular sinuses, not too dissimilar from the Ordovician ''

Lituites ''Lituites'' is an extinct nautiloid genus from the Middle Ordovician and type for the Lituitidae (a tarphycerid family) that in some more recent taxonomies has been classified with the orthocerids and listed under the order Lituitida. Foss ...

'', to which this genus is possibly allied. 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. ; Abstrac

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References

{{Taxonbar, from=Q48995742 Prehistoric nautiloid genera Silurian extinctions Silurian animals of Europe Cephalopod families Nautiloids Fossils of Sweden