Shell elongated, rather compressed, very inaequilateral, the umbones laying at about one sixth of the whole length of the shell, rather smooth, with numerous fine concentric striae; umbones little prominent, decorticated in the adult, in half grown shells with conspicuous radiating folds, extending more or less over the disk of the shell; moreover a few radiating grooves occupy the anterior part of the shell, and microscopic radiating wrinkles of the epidermis cover the posterior slope; epidermis reddish-brown in the adult, yellowish olive-green in the young ones. Upper margin slightly curved, anterior margin rounded, slightly protruded below; ventral margin incurved, posterior margin forming with the upper one a more or less obtuse angle, regularly curved above, rounded below and running with a very obtuse angle at the umbonal ridge, which is rather obtuse, into the ventral margin. Ligament long, narrow, yellowish-brown. Nacre bluish white, more iridescent near the posterior part, brownish near the umbones; scarcely more than one short, thick, crenated tooth in each valve; a small knob in the left valve may perhaps be considered as a second tooth; lateral lamellae: 2 in the left valve, one in the right one; they are elongated, slightly curved, rather thick.