Shells small to medium-sized, subglobose, with a flat spire, almost sunken in the large last whorl; sculpture made up of an assortment of spiral cords, growth marks and nodules, but one has to keep in mind that most of the species are nearly completely smooth; aperture semicircular, with a wide arcuate labrum and a straight columellar side, which often has indentations; columellar callus smooth to wrinkled; in some species, the terminals are projecting beyond the spire and the base. Operculum semilunar, calcareous, with a finger-shaped inner apophyse; nucleus anterior.
 

Shell medium-sized, solid, heavy; operculum thick, often granulose externally or with commarginal incremental marks, and often with a more or less marked thickening on the labial side.
 

In his work Malacologia del Mar Rosso, Pisa 1869, p.213, A. Issel proposes to establish the genus Smaragdia for « all the marine Neritina-like Nerites »… bearing in mind that by “Neritina” (sensu Montfort and not Lamarck) one may understand the small river-snails nowadays registered into the genus Theodoxus Montfort. The shell is small, thin-walled, smooth, usually patterned with contrasted colours. Issel notices that these animals can float upside down under the water surface, exactly « like the Limneids. »
 

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, the shell can vary considerably in shape, colours and pattern.