Shell semi-transparent, oval-conical, with six tumid whorls; often with globular or inflated body whorl and short spire. Ornamentation of fine spiral striae (not visible without magnification) and irregular growth lines, sometimes with costae on younger whorls. Costae, when present, usually weak, or even absent on youngest parts of last whorl, though a varix may be present. Costae not linked basally to a spiral ridge (cf. Rissoa parva ); less numerous on penultimate whorl (9-16) than in Pusillina inconspicua . Aperture oval, angulate, slightly drawn out adapically; peristome thin, often no more than a glaze where it crosses last whorl. A small umbilical chink (P. sarsii-drawing).
Up to 3 x 2.5 mm.
White, cream, or horn-coloured, with apical whorls often darker and sometimes with faint violet tinge (never dark purple/violet as in Pusillina inconspicua ). Penultimate and last whorls often with red-brown bands (lying between costae in costate shells), which may or may not connect basally to a spiral brown band on last whorl; usually a dark brown streak on columella.
Body similar to Rissoa parva , though with longer cephalic tentacles and longer, less cylindrical metapodial tentacle.
On rocky shores among weed, especially Codium and Zostera . Occasionally intertidal but more usually sublittoral to 15 m. Shows some tolerance to brackish waters.
Distributed from Mediterranean to northern Norway (Distr. P. sarsii).