Shell small, marginally sub-pentagonal, longer than wide, straight or slightly indented in front. Dorsal valve laterally gently convex, longitudinally concave, with a small median rib commencing at about the middle of the valve, and widening as it nears the front. Ventral valve very convex and keeled along the middle or divided longitudinally by a groove commencing at about half the length of the shell and extending to the front. Beak small, incurved, foramen minute, beak ridges strongly marked. Surface of valves smooth. In the interior of the dorsal valve, under the hinge-plate, a slightly elevated longitudinal septum or ridge extends to within a short distance of the frontal margin. To the hinge-plate are attached the principal stems of the loop, which, after giving off crural processes, extend to within a short distance of the front, where they become reflected so as to form the loop.