genus of small pelecypods (clams) especially characteristic of the Miocene Epoch (between 23.7 and 5.3 million years ago). The ornamentation of the shell includes prominent ribbing that extends from the apex to the broadly expanding margin. The ribs are broken up into a nodose pattern by fine lines, perpendicular to their axis of growth. Internally, the margin of the shell has a denticulate pattern formed by alternating ridges and troughs.