1. Diceras. One fossil species.

Only found in a fossil state; distinguished from the Chama by the hinge,

which is dissimilar, formed by a large thick tooth, concave in the

greater valve; summits very projecting; almost regular spiral

contortions.

_D. arietina._ The ram’s-horn Diceras.

Shell irregular, inequivalve, inequilateral, somewhat heart-shaped, with

divergent beaks.