There is only one freshwater genus of this family which occurs in Central and Northern Europe, and that is Theodoxus.
Theodoxus fluviatilis (Linnaeus, 1758)
Genus: THEODOXUS Montford, 1810
There are three species in Central and Northern Europe, namely:
T. transversalis (C. Pfeiffer, 1828)
T. fluviatilis (Linnaeus, 1758)
T. danubialis (C. Pfeiffer, 1828)
Of these, only T. fluviatilis occurs in Britain, but it is not frequent.
Theodoxus fluviatilis (Linnaeus, 1758)
Description: The shell is strong with thick walls. It is almost globular with a very low spire. The shell has very variable markings, but usually it is a pattern of pink-purple variegations on a yellow, brown or black background. There are only three whorls, each expanding rapidly. The aperture is large and half-moon shaped. It is closed by a calcareous operculum, often of a yellow or reddish colour. The columella is white and shelf-like.
Size: Height: 6 mm. Breadth: 11 mm.
Habitat: Found on stones, sunken wood, and walls of bridges, canals etc.; seldom in streams. Most commonly found in rivers and canals, but sometimes in lakes, particularly the wash-zone of calcareous lakes in Ireland. It is considered to be a hard water species.