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''Plectoceras'' is a nautiloid genus included in the tarphycerid suborder Barrandeocerina that lived during the Middle and Late Ordovician. It has been found widespread in the Middle and Upper Ordovician of North America. ''Plectoceras'' has a coiled, costate shell with a perforate (see-through) umblilcus; whorls are dorsally impressed by end of first volution but loosen in the adult stage with the mature body chamber diverging from the adjacent whorl. Costae (ribs) slope dorso-ventrally toward the apex and sutures are straight or have lateral lobes. The siphuncle is subventral, orthochoantic with short, straight necks and thin connecting rings. ''Plectoceras'' and its contemporary ''Barrandeoceras'' are both likely derived from the Tarphyceratidae; ''Plectoceras'' with its subventral siphuncle possibly from ''Campbelloceras'', ''Barrandeoceras'' with its subcentral siphuncle possibly from ''Centrotarphyceras''. ==References== *Rousseau H. Flower, 1984. Bodeiceras; a New Mohawkian Oxycone, with Revision of the Older Barrandeocerida and Discussion of the Status of the Order. Journal of Paleontology v. 58, no.6, pp 1372–1379, Nov. 1984. *Walter C Sweet, 1964 Nautiloidea-Barrandeocerida; Treatise on Invertebrate Paleontology Part K Mollusca 3. Geological Society of America and University of Kansas Press. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Plectoceras」の詳細全文を読む スポンサード リンク
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