Arietites

Arietites is a genus of massive, giant evolute, psiloceratacean ammonites in the family Arietitidae in which whorls are subquadrate and transversely ribbed and low keels in triplicate, separated by a pair of longitudinal grooves, run along the venter. Fossils are known world wide from the lower Sinemurian stage of the Lower Jurassic. Similar genera include Megarietites in which the keels are reduced and Epammonites in which the ribs are more closely spaced.