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 an inner apophyse and sometimes other extentions that are designed to anchor the adductor muscles; 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, the shell can vary considerably in shape, colours and pattern. Presence, on the operculum, of a labial margin made up of conchiolin.