(Eydoux and Souleyet, Ms.) (Gray, 1850)
Diagnosis: Shell transparent, triangular in outline. Ventral side vaulted, with 18 thin and straight ventral ribs; anterior part without ribs. Dorsal side convex, with well-developed central rib and latero-dorsal ribs; lock ribs small but rather thick, lock area well-developed. Characteristic nose in middle of the rostrum; owing to strong inner hump, which is continuous with underside of lip shoulder, central dorsal rib not continuous with rostrum. Gutter corners small. Sculpture of faint growth lines and faint transverse striation. Lateral spines perfectly straight, directed laterally, hooked and with sharp tips (in some populations lateral spines may be reduced). Caudal spine absent, no caudal spine mark since ventral and dorsal sides merge. Dorsal lip with notch, gutter with inner hump, no constriction. Ventral lip moderately sized with clear median depression. Shell length 4.8 mm, width 3.7 mm.