Diagnostic characters
Shell solid and glossy, hemispherical, with low spire, the last whorl forming most of the shell; aperture D-shaped, half blocked by an expansion of the columellar region.
Other characters
There are three whorls, the sutures of variable depth but distinct. The only ornament is fine growth lines. There is no umbilicus. The shell is usually dark in colour with a pattern of yellow-white streaks, and there are often three darker areas separated by two lighter ones on the last whorl (Neumann. 1959); the dark areas are subsutural. peripheral and basal in position. The columellar plate is white or yellow with a darker edge along the inner lip. Up to 8mm high, 8mm broad; last whorl occupies 95% of shell height, aperture 90%.
There is a broad snout, its edge lobulate. At its base lie the cephalic tentacles, capable of great extension and retraction. An eye on a separate eye stalk lies alongside each. The mantle edge is thick and papillated. In males a flattened penis with an open seminal groove lies behind the right tentacle. The foot has an epipodial fold on each side but this carries no tentacles; that on the left is joined to the eye stalk. The operculum, partly calcified, is D-shaped and has an internal peg. The flesh is white-yellow, with many black flecks, particularly on the snout, the mantle edge, the foot.
T. fluviatilis lives mainly in rivers, hiding under stones or wood by day, foraging by night on plants and detritus (Fromming. 1956). It also occurs in canals and the littoral region of lakes where there is movement in the water. The animals are limited to hard waters, preferring a calcium content of 20-30mg. They tolerate a moderately high salinity, up to 179ft in the Loch of Stenness. Orkney (Nicol, 1938). The species occurs generally throughout Europe except for the most northern parts, the Iberian peninsula and the central Danube basin. In the British Isles it is absent from Devon and Cornwall in England, occurs only in Glamorgan in Wales, in the Lochs of Stenness and Harray (Orkney) in Scotland (Boycott, 1936), and in the lime¬stone districts of Ireland.
Breeding begins late spring to early summer with spawning from early summer to early autumn. Females lay capsules on wood or stones or the shells of other animals. Each capsule consists of two halves, approximately hemispherical, the lower attached to the substratum, the upper fitting over its mouth. They are made of mucous material and the upper is hardened by a coat of sand grains, diatom cases and similar objects derived from the faeces and stored in a so-called crystal sac until used (Fretter. 1946). Each capsule may contain up to about 150 eggs, but only one develops successfully, hatching as a juvenile snail, the others being used as its food. In brackish water the number of eggs in a capsule drops to 50-60 (Bondesen. 1940).