Rioceras

Rioceras is an ellesmerocerid genus and member of the Baltoceratidae that produced small slender shells, mostly with a circular cross section but varying specimen to specimen from slightly compressed to slightly depressed. Apeces are slightly endogastric, a possible take over from Ectenolites, or perhaps even Eremoceras.

The siphuncle, located closed to the ventral margin, is relatively large, 1/4 to 1/3 the shell diameter. Septal necks are short, connecting rings moderately thick. Chambers are short, sutures straight and transverse. Baltoceras, somewhat younger in age, has a more slender shell. Rioceras fossils have been found at various locations in southern New Mexico

References
 * Rousseau H. Flower 1964, The Nautiloid Order Ellesmeroceratida (Cephalopoda). New Mexico Bureau of Mines and Mineral Resources Memoir 12.