Deltoceras Hyatt 1894

Genus Deltoceras Hyatt, 1894 Type species

Deltoceras planum Hyatt, 1894, from strata of the St. George Group (Lower Ordovician) at Port au Choix, north side, western Newfoundland, Canada; by original designation.

Rapidly expanding evolute conch possessing whorls with a compressed cross section with very shallow or lacking zone of impression; adoral portion of conch divergent from preceding whorl at maturity; no conspicuous shell sculpture; sutures form weak lateral lobes; siphuncle subventral in position (adopted from Evans 2011).

The exact location of the only known specimen of the type species is given in Hyatt (1894: 450) as “Port au Choix, north side” without specifying wether this relates to the Port au Choix peninsula or the town of Port au Choix. Deltoceras is listed under doubtful taxa in Teichert (1964), but Evans (2011) revived the genus and provided a concise diagnosis adopted from Ulrich et al. (1942), which is used herein. The genus diagnosis is slightly emended and expanded, herein, from being exclusively without dorsal impression zone to also having a very shallow impressive zone, in order to include Deltoceras beluga sp. nov. We justify this emendation also based on the poor knowledge on the details of the single specimen of the type species, which may also during earlier growth stages have a slightly involute conch curvature. The emphasis of this genus now is the combination of strongly compressed smooth whorls with a subventrally positioned siphuncle.