Diceras

Diceras, a remarkable extinct genus of Lamellibranch mollusca, allied to the modern Chama. The two valves were almost equal, and the "umbo," or beak, is of great size in each valve, and is coiled as a loose spiral; hence, single valves resemble the shells of Gastropoda. The genus occurs in the Oolitic rocks of the Continent. A species of a closely allied genus occurs in the Lower Greensand of the South of England; it is known as Requienia lonsdalei.