Diagnostic characters
Shell small, semitransparent, glossy, with blunt tip; whorls tumid and flattened below the sutures which are deep and gutter-like; last whorl large. Pallial tentacle absent. Snout dark with pale tip; an orange-yellow spot behind each eye.
Other characters
There are 5-6 whorls forming a rather broad, squat shell with an apical angle usually about 66°, so much greater than that of species of Hydrobia or Potamopyrgus . There is a well marked umbilicus. Pale horn colour. Up to 4 mm high, 2 mm broad; last whorl occupies about three quarters of the shell height, the aperture about half.
The animal has the same external features as most hydrobiids. The tentacles are long and pale except for a central longitudinal brown band.
P. confusa (= Mercuria similis) occurs in places where the water is brackish but nearly fresh, mainly in the upper tidal reaches of quiet-running rivers. It may be found on plants or the muddy edges or bottoms of water channels. Though widespread in Iberia and France it is rare in the British Isles but has been recorded from the River Arun in Sussex, the Thames estuary (Harris, 1985), some East Anglian rivers and those of southern Ireland.