A small, brown, semitransparent and nearly smooth shell of four whorls, with a low spire, deep umbilicus and nearly circular aperture. The whorls meet at deep sutures and are marked with numerous fine growth lines. The outer lip in young animals has often a projecting edge of uncalcified periostracum. In basal view the inner lip of the aperture does not bound or cover the umbilicus. The shell is chestnut brown, and often paler near the aperture. 0.75 mm high, 1.5 mm broad; last whorl occupies about 95% of shell height, aperture about three quarters.
The flesh is grey with yellow spots, with yellow around the bases of the tentacles and under the operculum.
S. planorbis may be found in pools and on weeds on the lower half of rocky shores and it also occurs to depths of 70 m. It may be extremely abun¬dant, especially in summer, feeding on the weeds over which it crawls, and on their epiphytes (Fretter, 1948). It occurs practically everywhere in the British Isles where the shores are suitable, and ranges from the Mediterranean and the Azores to the Arctic on the eastern side of the Atlantic, through Iceland and Greenland to Arctic Canada and thence south to Florida.
Breeding may occur at any time of the year but is maximal in spring. Spherical or ovoid egg capsules are abundant on Cladophora or other filamen¬tous algae; they measure 350-450 µm and contain one or two eggs which develop to young snails in 3-4 weeks (Lebour, 1937).