The genus inhabits freshwater and mesohaline environments. « Shell free, univalve, with a regular spire, flattened; no umbilicus; aperture entire, round, widened, perpendicular to the horizon; outer lip sharp; no tooth at this lip nor at the columella. […] The Theodoxes being devoid of teeth, neither at the columella nor at the outer lip, could not be incorporated with the Nerites, or in the genus Clithon; this is the reason why we established for them a peculiar genus. » – P. D. de Montfort: Conchyliologie systématique vol.II, Paris 1810, p.351. In some species such as fluviatilis or baeticus-meridionalis, the shell can vary considerably in shape, colours and pattern. Presence, on the operculum, of a labial margin made up of conchiolin, and of a feather-shaped apophysis located, on the inner face, near the columellar edge.