Shell generally higher than wide, rather delicate. Valves inequilateral, left valve more convex than right one. Pronounced (primary) ribs left valve single and hollow, hiding riblets formed by rows of small tubercles, they become visible when ribs are damaged. Secondary ribs also consist of rows of small tubercles which, however, are not hidden. In between primary and secondary ribs fine grooves may be observed. All ribs of right valve equally pronounced, but clustered on broad plicae that coincide with the interspaces between the pronounced ribs of left valve. Ribs appear gradually at equal distances from apex. Anterior auricles larger than posterior, with a reticulate structure carrying tubercles at each crossing point. In between this structure fine grooves are visible. Posterior auricles with tuberculous ribs only. Remarks. - Actually in Europe this genus is represented by 2 species, but as a third one is known from the Azores to Mauretania, and southwards along West Africa, it is included in this study also.