(de Blainville, 1821)
Diagnosis: Shell hyaline, triangular, brown, especially near ventral lip border and centre of dorsal and ventral sides. Ventral side vaulted. Seventeen thin, linear ventral ribs composed of small tubercles. Dorsal side slightly convex, with well-developed central rib, lateral ribs and lock ribs. Lock area small with blunt or sometimes sharp main tubercle and small link. Lunar and minor tubercles typical. Second lock mechanism present. Lateral lines slightly convex, flanks poorly developed. Growth lines strong on both ventral and dorsal sides. Sharp lateral spines slightly bent, protruding laterally and slightly hooked; gutter corners large. Caudal spine and caudal spine mark absent. Dorsal lip with notch and gutter but no constriction. Ventral lip moderately sized with distinct median depression. Only traces of lip flaps; lip bellies and lip shoulders small. Aperture oval, large, lip angle about 135°, lock angle 77°, side angle 66°. Aperture forms two long lateral slits directed posteriorly, aperture rim sickle-shaped in dorsal view. Caudal fold about 1.60 mm long, nearly straight, left and right folds form angle with each other. Hump absent. Protoconch II area projecting moderately, with curved caudal joint, of about 1.28 mm. Juveniles dorso-ventrally compressed, spoon-shaped in ventral view with straight shell except for posteriormost section that bends slightly dorsally. In older juveniles caudal spine is discarded and rounded shell composed of two valves is left, initially open caudally. Shell length 7.2 mm, width 6.8 mm.