The families of British non-marine molluscs (slugs, snails and mussels)


L. Watson and M. J. Dallwitz


Morphology. Snails, with a conspicuous, spiral, univalve shell.

The shell operculate (the operculum calcareous and solid, with an apophysis); rising-spiral; 3 whorled; typically dextral; wider than high; about (7.3–)8–11.6 mm high; (8.25–)9–13 mm wide; height about 0.9 x the width; with the body whorl predominating and the spire small and short. The height of the spire about 0.15 x that of the shell. The shell semi-lunate, very asymmetric about the vertical axis, with the spire much displaced to one side. The shell very asymmetic about its vertical axis, with a large body whorl and a small, displaced spire; deeply sutured (at the junction of the body whorl and the spire). The whorls shouldered. The aperture with neither teeth nor calluses. The shell without an umbilicus. The shell thick-lipped; opaque; basically whitish or yellow to brown or even black; conspicuously colour-patterned (with irregular purple, pink or white variegations, or transverse bands, or spirally marked with three dark bands).

General biology, ecology. Freshwater aquatic. Breathing via a single gill attached within the mantle cavity. Restricted to well oxygenated waters, in rivers, canals and lakes.

The individuals either male or female (not hermaphrodite).

Classification. Gastropoda; Prosobranchia.

British representation. Theodoxus (1, “River Nerite”).

Illustrations. • Theodoxus fluviatilis (Reeve). • Theodoxus fluviatilis, with other Gastropoda-Prosobranchia (Adams).

To view the illustrations with detailed captions, go to the interactive key. This also offers full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, and distributions of character states within any set of taxa.

Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 2005 onwards. The families of British non-marine molluscs (slugs, snails and mussels). Version: 4th January 2012.’.