(Alder, 1844)
Description (shell):
Shell oval-conical, with five or six semi-transparent, glossy, slightly tumid whorls. Spire profile variable; flat-sided to appreciably cyrtoconoid. Sculpture of fine spiral striae, most prominent at periphery of last whorl; with or without numerous (18-30), fine, blunt costae on all but upper three whorls (P. inconspicua). Interaction of costae and striae produces a fine reticulate pattern, most obvious at periphery of last whorl. A small umbilical groove but no umbilicus. Aperture oval, with a thickened peristome and a distinct anal sinus (P. inconspicua-drawing).
Size:
Up to 3 x 2 mm.
Colour:
Shell white or horn-coloured, with deep purple apex. Younger whorls often with a series of red-brown streaks below suture; peristome red-brown.
Animal:
Resembles Rissoa parva but the finger-shaped metapodial tentacle is long and the penis is flattened and lacks a terminal filamentous part. Body is whitish with numerous yellow spots, one large one by each eye, two dark lines along each side of the foot and black opercular lobes (P. inconspicua-animal).
Habitat:
On weeds in low shore rock pools, though more common sublittorally on algae and shell gravel to 100 m. Shows limited tolerance of brackish waters.
Distribution:
Distributed from the Azores and Mediterranean to Arctic waters; in the Baltic as far as Lubeck Bay (Distr. P. inconspicua).