C. t. crassa, subrhombica, vix inequivalvi, perinequilater-ali; margine antice rotundata, aliunde rectilineari, postice infra acuminata: alba: costis magnis, subacutis, concentricis, postice minoribus; lineis radiantibus tuberculatis, microseop-icis; tuberculis deciduis; valvis ambobus similiter insculptis; apicibus prominulis, vix involutis: umbonibus planulatis, postice valde angulatis: dentibus parvis.

Shell thick, subrhombic, slightly inequivalve, very inequi-lateral; anteriorly well rounded, with the other margins nearly or quite rectilinear; posteriorly and inferiorly acumi¬nate: white: with large blunt edged concentric ribs, which are more slender in the posterior area; with microscopic radiating lines of deciduous tubercles, which are visible only under a good magnifier; with both valves sculptured alike: beaks prominent, slightly involute: umbones flattened, with a stout posterior angle: teeth rather small. Under a good magnifier a crystalline structure is very conspicuous. Length .44 inch; height .26 inch; breadth .18 inch.