genus of pelecypods (clams) abundant during the Eocene Epoch (the Eocene Epoch began 57.8 million years ago and ended 36.6 million years ago). The shell, composed of two halves (valves), is distinctive in form and generally large. Transverse ribs radiate from the apex of the valves and are broken by a series of concentric growth rings. Internally, the valves are marked by distinctive nodes along the edge and thickenings that form raised bars at the apex; these form the surfaces along which the valves articulate.