This species appears to vary somewhat in appearance, but is usually transversely semi-cylindrical, the hinge-line being at the same time the widest portion of the shell. The ventral valve is very much vaulted, and usually longitudinally divided by a median depression or sinus of variable depth. The beak is small, and does not overlie the hinge-line, while the ears are of moderate dimensions and clearly defined. Exteriorly the surface is ornamented with numerous longitudinal ribs, which increase in number by means of occasional intercalations; certain ribs will also disappear before having attained the margin; while again two will sometimes unite, so as to constitute but a single rib. The costæ are very often of unequal width, rounded, or flattened, and sometimes will rapidly increase in width as they extend towards the margin: concentric wrinkles are also observable upon the auriculate portions of the valve, and a row of long tubular spines may be seen close to the cardinal edge, as well as upon the lateral portions of the beak; spines of smaller proportions project likewise here and there from the ribs themselves, while the longitudinal costæ are closely intersected or decussated to some distance from the extremity of the beak by numerous undulating concentric lines.