Arietites; Print; 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.