''Plectoceras'' is a genus of nautiloids included in the tarphycerid suborder

Barrandeocerina The Barrandeocerina comprise a suborder of Early Paleozoic nautiloid cephalopods, primitively coiled but later forms may be cyrtoconic, gyroconic, torticonic, and even breviconic, all having empty siphuncles with thin connecting rings. The Bar ...

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, orthochoanitic with short, straight necks and thin connecting rings. ''Plectoceras'' and its contemporary ''

Barrandeoceras ''Barrandeoceras'' is a large, coiled, Middle Ordovician nautiloid cephalopod and part of the Tarphycerida. The shell is serpenticonic with whorls touching but not embracing. The adult body chamber becomes freed of the preceding whorl, a rather ...

'' are both likely derived from the

Tarphyceratidae The Tarphyceratidae are tightly coiled, evolute Tarphycerida with ventral siphuncles. The dorsum is characteristically impressed where the whorl presses against the venter of the previous. The Tarphyceratidae are derived from ''Bassleroceras'' or ...

; ''Plectoceras'' with its subventral siphuncle possibly from ''

Campbelloceras ''Campbelloceras'' is a tarphyceratid nautiloid known from the Lower Ordovician, Upper Canadian Epoch of North America, where it is widespread. ''Campbelloceras'' was named by Ulrich and Foerste in 1936. The shell of ''Campbelloceras'' has a ci ...

'', ''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. {{Taxonbar, from=Q7204383 Prehistoric nautiloid genera Ordovician cephalopods Ordovician cephalopods of North America Middle Ordovician first appearances Late Ordovician extinctions Paleozoic life of Nunavut Tarphycerida