Shell aragonitic, usually small, inequilateral; interior nacreous. Inflated, equivalve, rounded-trigonal, with truncate posterior end. Periostracum adherent, usually dark colored. Hinge plate strong, with series of V-shaped teeth separated by resilifer. Ligament internal, amphidetic. Dimyarian; pallial line weak; pallial sinus absent. Gills small, oblique to horizontal, with numerous filaments, posterior end free. Labial palps and palp probóscides large. Mypobranchial gland present. Intestine coiling on right side of stomach.