Hesperoceras is an orthocerid

Orthocerida

Orthocerida is an order of extinct nautiloid cephalopods also known as the Michelinocerda that lived from the Early Ordovician possibly to the Late Triassic . A fossil found in the Caucasus suggests they may even have survived until the Early Cretaceous...

 cephalopod belonging to the subfamily Michelinoceratinae which comes from the lower Mississippian of New Mexico

New Mexico

New Mexico is a state located in the southwest and western regions of the United States. New Mexico is also usually considered one of the Mountain States. With a population density of 16 per square mile, New Mexico is the sixth-most sparsely inhabited U.S...

Hesperoceras has a straight, orthoconic, shell with a subrectangular cross section. Sutures are straight and transverse on the narrow sides but form broad rounded lobes on the wide sides. The siphuncle is small and central.

References

• Sweet, Walter C. 1964. Nautiloidea-Orthocerida. Treatise on Invertebrate Paleontology, Part K. Geological Soc of America, and Univ Kansas Press.