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 animal with two pairs of tentacles. Eyes at the tips of the posterior tentacles.

The shell inoperculate; rising-spiral (but only slightly raised in Vallonia); 3–3.25 whorled (Vallonia), or 4 whorled (Acanthinula), or 5.5–6 whorled (Spermodea); typically dextral; 2–2.7 mm in its maximum dimension; about as high as wide (Acanthinula), or about as high as wide to wider than high (Spermodea), or wider than high (Vallonia); 2 mm high (Acanthinula), or 2–2.7 mm high (Spermodea, Vallonia); 2 mm wide (Acanthinula, Spermodea), or 1.25–1.4 mm wide (Vallonia); height about 0.5–0.55 x the width (Vallonia), or 0.81 x the width (Spermodea), or 1.13 x the width (Acanthinula); with the body whorl predominating and the spire small and short (in Vallonia), or high-spired and tapered gradually from the body whorl. The height of the spire about 0.14–1.18 x that of the shell (Vallonia), or 0.33 x that of the shell (Acanthinula and Spermodea). The spire when raised, obtuse. The shell when wider than high, discoid, or cochleate (in Vallonia), or cochleate to globose (in Spermodea); when higher than wide, broadly inverted-pyriform (in Acanthinula). The body whorl very strongly convex. The whorls neither shouldered nor keeled to shouldered, or keeled (in Acanthinula); exhibiting a median whorl of spines (in Acanthinula only, the spines derived via prolongations from the transverse ridges, along the keel), or without spines; conspicuously and regularly transversely ridged across the whorls (V. costata, Spermodea lamellata, and Acanthinula), or not conspicuously transversely ridged. The aperture round; with neither teeth nor calluses. The shell with an umbilicus. The umbilicus large and wide (open). The shell thick-lipped (the mouth edge more or less expanded to form a flat lip); thin and translucent, or opaque; greyish white or translucent (Vallonia), pale golden brown (Spermodea), or brown (unweathered Acanthinula); plain.

General biology, ecology. Terrestrial. In diverse habitats.

Hermaphrodite. Courtship involving exchanges of ‘love darts’ prior to mating (the Vallonia species being equipped with a dart sac secreting a straight, simple dart: cf. Ellis), or not involving ‘love darts’.

Classification. Gastropoda; Pulmonata.

British representation. Acanthinula (1, “Prickly snail”), Spermodea (1, “Plated snail”), Vallonia (3, “Grass snails”).

Illustrations. • Vallonia costata, V. excentrica, V. pulchella (Ellis). • Acanthinula aculeata, Spermodea lamellata (Ellis). • Acanthinula aculeata, Spermodea lamellata, Vallonia pulchella, with Discidae, Euconulidae, Punctidae, Pyramidulidae, Vitrinidae (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.’.