Taxa named by Alpheus Hyatt

Arnioceras is an extinct genus of large, evolute, discoidal ammonite from the Lower Jurassic. The shell is normally coiled so that all whorls are exposed. Sides bear strong sharp ribs that are straight until reaching the ventrolateral edge where they swing forward and fade. The rim (venter) is keeled and free of grooves.

Didymoceras is an extinct genus of ammonite cephalopod from the Late Cretaceous epoch (approximately 76 Ma). It is one of the most bizarrely shaped genera, with a shell that spirals upwards into a loose, hooked tip. It is thought to have drifted in the water vertically, moving up and down. The genus name comes from Ancient Greek δίδυμος (dídumos), meaning "twin", and κέρας (céras), meaning "horn".