Cyrtorizoceras Hyatt in Zittel, 1900

Kröger, Björn, 2013, The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden, European Journal of Taxonomy 41, pp. 1-110 : 20-21

Genus Cyrtorizoceras Hyatt in Zittel, 1900

Type species

Cyrtoceras minneapolis Clarke, 1897 , Minneapolis, Minnesota, USA, Platteville Formation, early to mid Katian, Late Ordovician; by original designation.

Diagnosis

Compressed cyrtocones with conchs expanding constantly on anti- and prosiphuncular side; body chamber slightly contracted; aperture with deep hyponomic sinus; surface ornamented with obscure or prominent transverse costae; siphuncle close to conch margin on convex side of conch curvature, slender; siphuncular segments slightly expanded, probably empty. (Adopted from Sweet 1964a.)

Comparison

Cyrtorizoceras is similar to Richardsonoceras Foerste, 1932 in having a gradually enlarging cyrtoconic conch with a curved mature body chamber. Cyrtorizoceras differs from Richardsonoceras in having

a generally less curved conch, higher angles of expansion and a mature body chamber with a more pronounced, angular sinus on the prosiphuncular side of the mature body chamber.

Remarks

This is a species rich genus with more than 35 species described, mostly known from the Late Ordovician of North America. The genus is in need of revision, as it probably encompasses several distinct and perhaps distantly related groups of cyrtoconic oncoceratids, all possessing a simple, non gibbous body chamber.