Arnioceras semicostatum Ammonite, from Dorset, Monmouth Beach, Lyme Regis, UK (REF:ASA1)

Arnioceras is an extinct genus of large, evolute, discoidal ammonite from the Lower Jurassic.

The shell is normal coiled so that all whorles 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, (April 5, 1838 – January 15, 1902) an American zoologist and palaeontologist.

Arnioceras distribution is fairly worldwide, and have been found in Europe, South Asia, and North and South America.