Arietitidae

Asteroceras (from Ancient Greek ἀστήρ (astḗr), meaning "star", and κέρας (kéras), meaning "horn") is an extinct genus of cephalopod belonging to the Ammonite subclass. These fast-moving nektonic carnivores lived during the Triassic and Jurassic periods (from 205.6 to 189.6 Ma).

thumb|right|Arietites bucklandi fossil at the Geological Museum in Copenhagen

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.