Shell solid, subquadrate to ovate. Posterior end produced, set off by radial furrow. Sculpture absent, or of irregular incremental striae. Periostracum thick, light brown to black, adherent. Hinge plate narrow. Resilifer small, triangular, projecting; ligament amphidetic. Pallial sinus small to obsolete. Inhalant and exhalant siphons fused together; inhalant siphon open ventrally.
Extending to the Paleocene, the genus is represented by four variable species in the northeastern Pacific. The genus is named for Margaret Cavendish Bentinck, second Duchess of Portland, an early British shell collector; the gender is feminine.