''Ulrichoceras'' is recognised as the basal cyrtogomphoceratid genus, which is the source for the rest of the

Cyrtogomphoceratidae The Cyrtogomphoceratidae are a family in the cephalopod order Discosorida that comprises genera commonly with compressed, endogastrically curved shells. Siphuncles lie close to the ventral side, segments are broadly inflated, connecting rings th ...

as well as for the

Westonoceratidae Westonoceratidae are exogastric, mostly compressed, Discosorida of moderate size from the Middle Ordovician to the Lower Silurian. The siphuncle is typically close to the convexly curved outer margin of the phragmocone – the chambered part o ...

. The Cyrtogomphoceratidae are endogastric, the Westonoceratidae, exogastric. ''Ulrichoceras'' has a cyrtoconic, endogastric shell that expands fairly rapidly to the middle of the body chamber, then contracts toward the

aperture In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane. An opt ...

. The cross section is slightly depressed, atypical for cyrtogomphoceratids but not so for Reudemannoceratids; dorsal and ventro lateral sides are broadly rounded, the venter moderately flattened. Sutures have broad ventral lobes. The

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

is between the center and the venter; segments are short and not strongly contracted at the septal foremina, connecting rings are thickened and have enlarged bullettes. ''Ulrichoceras'', named by Foerste, 1925, type ''U. beloitense'', comes from the Middle Ordovician of Wisconsin, U.S.A. It is intermediary in form between '' Reudemanoceras'' and the more derived, M -U Ordovician, '' Strandoceras''.

References

* Flower, R H and Teichert, C; 1957. The Cephalopod Order Discosorida. University of Kansas Paleontological Contributions. Mollusca, Article 6, pp 1–144. * Teicher, C. 1964. Nautiloidea-Discosorida.

Treatise on Invertebrate Paleontology The ''Treatise on Invertebrate Paleontology'' (or ''TIP'') published by the Geological Society of America and the University of Kansas Press, is a definitive multi-authored work of some 50 volumes, written by more than 300 paleontologists, and co ...

, Part K. Teichert and Moore, eds. Prehistoric nautiloid genera Discosorida {{paleo-Nautiloidea-stub

See also

References