Wikipedia
Chouteauoceras
Chouteauceras is an openly coiled, gyroconic, nautiloid cephalopod from the Mississippian of North America belonging to the Nautilid family Trigonoceratidae, and superfamily Trigonocerataceae.
The whorl section of Chouteauceras is ovate, higher than wide. The surface is covered with numerous longitudinal ridges and fine growth lines. The suture has broad rounded lateral lobes and dorsal and ventral saddles. The siphuncle is small, subcentral.