The families of British non-marine molluscs (slugs, snails and mussels)
Morphology. Snails, with a conspicuous, spiral, univalve shell.
The animal with two pairs of tentacles. Eyes present, or absent (Helicodiscus singleyanus being subterranean and blind); when present, at the tips of the posterior tentacles. The jaw not as in Zonitidae and Limacidae; of a single piece.
The shell inoperculate; rising-spiral; 3.5–4 whorled (Helicodiscus), or 4–6 whorled (Discus); typically dextral; 1.8–2.5 mm in its maximum dimension (Helicodiscus), or 5.5–7 mm in its maximum dimension (Discus); wider than high; 1.8–2.5 mm wide (Helicodiscus), or 5.5–7 mm wide (Discus); height about 0.43–0.65 x the width; with the body whorl predominating and the spire small and short to high-spired and tapered gradually from the body whorl. The height of the spire about 0.15–0.29 x that of the shell. The spire obtuse. The shell discoid, or cochleate; deeply sutured. The body whorl moderately convex to very strongly convex. The whorls neither shouldered nor keeled, or shouldered. The aperture with neither teeth nor calluses. The shell with an umbilicus. The umbilicus large and wide. The shell thin-lipped (the mouth edge simple, not deflected); thin and translucent to opaque; yellowish- or greenish-brown; conspicuously colour-patterned (that of D. rotundatus with regularly spaced reddish-brown transverse stripes across the whorls), or plain (in the rest).
General biology, ecology. Terrestrial.
Classification. Gastropoda; Pulmonata.
British representation. Discus (2), Helicodiscus (1).
Illustrations. • Discus rotundatus (Taylor). • Discus rotundatus, with Euconulidae, Punctidae, Pyramidulidae, Valloniidae, 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. http://delta-intkey.com’.