Arnioceras

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.
"Arnioceras", named by Alpheus Hyatt, is included in the arietitid family of the Psiloceratoidea, an ammonitid supefamily. Its distribution is fairly worldwide, having been found in Europe, south Asia, and North and South America.