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

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L. Watson and M. J. Dallwitz

Conventional key

If you are serious about identification, you should use the interactive key. It is far superior, with flexible choice of characters, character notes and illustrations, error tolerance, full taxon descriptions, and taxon illustrations.

1(0).
  • snails, with a conspicuous, spiral, univalve shell ... 2
  • limpets, with a conspicuous, protective patelliform shell bearing only a vestigial spire... 46
  • slug-like, but with a small, posterior external shell covering the mantle ... Testacellidae
  • slugs, with no external shell... 47
  • mussels, enclosed by a hinged, bivalve shell... 50

2(1).
  • The shell higher than wide ... 3
  • The shell about as high as wide... 24
  • The shell wider than high... 27

3(2).
  • The shell typically sinistral ... 4
  • The shell typically dextral... 6

4(3).
  • Eyes at the tips of the posterior tentacles; terrestrial ... 5
  • Eyes at the bases of the tentacles; freshwater aquatic ... Physidae

5(4).
  • The animal with one pair of tentacles only; the shell thin-lipped; the shell 1 to 4.5 mm high; height of the shell about 1 to 2.2 x its width (shell height divided by maximum width) ... Vertiginidae
  • The animal with two pairs of tentacles; the shell thick-lipped; the shell 7.5 to 20 mm high; height of the shell about 3 to 5 x its width (shell height divided by maximum width) ... Clausiliidae

6(3).
  • The shell operculate; the individuals either male or female (not hermaphrodite); Prosobranchia ... 7
  • The shell inoperculate; hermaphrodite; Pulmonata... 14

7(6).
  • The shell up to 3.5 mm in its maximum dimension ... 8
  • The shell 3.5 to 10 mm in its maximum dimension... 10
  • The shell 10 to 20 mm in its maximum dimension... 13
  • The shell 20 to 50 mm in its maximum dimension ... Viviparidae

8(7).
  • The shell thick-lipped; the shell not markedly striated across the whorls ... 9
  • The shell thin-lipped; the shell conspicuously and regularly striated across the whorls (i.e., along its length) ... Aciculidae

9(8).
  • The shell with an umbilicus; @pÐml> ÅÄ; breathing via a single gill attached within the mantle cavity ... Hydrobiidae
  • The shell without an umbilicus; ; breathing air directly via the lung-like mantle cavity ... Assimineidae

10(7).
  • The shell with the body whorl predominating and the spire small and short ... 11
  • The shell high-spired and tapered gradually from the body whorl... 12

11(10).
  • terrestrial ... Assimineidae
  • freshwater aquatic ... Bithyniidae
  • semi-marine (in saltmarshes and brackish water, or around the high tide zone) ... Assimineidae

12(10).
  • The shell with an umbilicus; @pÐml> ÅÄ; breathing via a single gill attached within the mantle cavity ... Hydrobiidae
  • The shell without an umbilicus; ; breathing air directly via the lung-like mantle cavity ... Assimineidae

13(7).
  • terrestrial; breathing air directly via the lung-like mantle cavity ... Pomatiasidae
  • freshwater aquatic; breathing via a single gill attached within the mantle cavity ... Bithyniidae

14(6).
  • Eyes at the tips of the posterior tentacles ... 15
  • Eyes at the bases of the tentacles... 22

15(14).
  • The shell thick-lipped ... 16
  • The shell thin-lipped... 20

16(15).
  • The shell with an umbilicus ... 17
  • The shell without an umbilicus... 19

17(16).
  • Courtship involving exchanges of ‘love darts’ prior to mating ... Helicidae
  • Courtship not involving ‘love darts’... 18

18(17).
  • The shell ovoid-symmetric; the shell 1 to 4.5 mm high; the shell 1 to 3 mm wide; height of the shell about 1 to 2.2 x its width (shell height divided by maximum width) ... Pupillidae
  • The shell tear-shaped (cf. inverted-pyriform, but more gradually contracted into the spire); the shell 7.5 to 20 mm high; the shell 3 to 8 mm wide; height of the shell about 2.2 to 3 x its width (shell height divided by maximum width) ... Enidae

19(16).
  • The spire acute; the shell conspicuously and regularly striated across the whorls (i.e., along its length); the shell relatively thick and opaque ... Chondrinidae
  • The spire obtuse; the shell not markedly striated across the whorls; the shell thin and translucent ... Cochlicopidae

20(15).
  • The shell cylindrical ... Vertiginidae
  • The shell ovoid-symmetric... 21
  • The shell ovoid-asymmetric ... Succineidae
  • The shell inverted-pyriform (pear-shaped) ... Succineidae
  • The shell tear-shaped (cf. inverted-pyriform, but more gradually contracted into the spire) ... Ferussaciidae

21(20).
  • The animal with one pair of tentacles only ... Vertiginidae
  • The animal with two pairs of tentacles ... Pupillidae

22(14).
  • The aperture bearing teeth ... Ellobiidae
  • The aperture with neither teeth nor calluses... 23

23(22).
  • The animal with one pair of tentacles only; the spire acute; the shell deeply sutured; eyes present; lifestyle not subterranean ... Lymnaeidae
  • The animal with two pairs of tentacles; the spire obtuse; the shell shallowly sutured; eyes absent; lifestyle subterranean ... Ferussaciidae

24(2).
  • Eyes at the tips of the posterior tentacles ... 25
  • Eyes at the bases of the tentacles... 26

25(24).
  • The shell up to 3.5 mm in its maximum dimension ... Valloniidae
  • The shell 3.5 to 10 mm in its maximum dimension ... Helicidae
  • The shell 10 to 20 mm in its maximum dimension ... Helicidae
  • The shell 20 to 50 mm in its maximum dimension ... Helicidae
  • The shell 50 to 85 mm in its maximum dimension ... Helicidae

26(24).
  • The shell operculate; breathing via a single gill attached within the mantle cavity; Prosobranchia ... Valvatidae
  • The shell inoperculate; breathing air directly via the lung-like mantle cavity; Pulmonata ... Lymnaeidae

27(2).
  • The shell typically sinistral ... Planorbidae
  • The shell typically dextral... 28

28(27).
  • The shell up to 3.5 mm in its maximum dimension ... 29
  • The shell 3.5 to 10 mm in its maximum dimension... 36
  • The shell 10 to 20 mm in its maximum dimension... 42
  • The shell 20 to 50 mm in its maximum dimension... 45
  • The shell 50 to 85 mm in its maximum dimension ... Helicidae

29(28).
  • Eyes at the tips of the posterior tentacles; the shell inoperculate; terrestrial; breathing air directly via the lung-like mantle cavity; Pulmonata ... 30
  • Eyes at the bases of the tentacles; the shell operculate; freshwater aquatic; breathing via a single gill attached within the mantle cavity; Prosobranchia ... Valvatidae

30(29).
  • The shell thick-lipped ... Valloniidae
  • The shell thin-lipped... 31

31(30).
  • The shell with the body whorl predominating and the spire small and short ... 32
  • The shell high-spired and tapered gradually from the body whorl... 34

32(31).
  • The jaw arcuate, with a vertical central keel ending in a median projection on the lower edge ... Zonitidae
  • The jaw not as in Zonitidae and Limacidae... 33

33(32).
  • The jaw of a single piece ... Discidae
  • The jaw of 19 separate plates ... Punctidae

34(31).
  • The umbilicus small ... Euconulidae
  • The umbilicus large and wide... 35

35(34).
  • viviparous ... Pyramidulidae
  • oviparous ... Discidae

36(28).
  • The mantle partly covering the shell when the animal is in motion ... Vitrinidae
  • The mantle not extending over the shell... 37

37(36).
  • The shell operculate; freshwater aquatic; breathing via a single gill attached within the mantle cavity; Prosobranchia ... 38
  • The shell inoperculate; terrestrial; breathing air directly via the lung-like mantle cavity; Pulmonata... 39

38(37).
  • The whorls neither shouldered nor keeled; hermaphrodite ... Valvatidae
  • The whorls shouldered; the individuals either male or female (not hermaphrodite) ... Neritidae

39(37).
  • The shell thick-lipped ... Helicidae
  • The shell thin-lipped... 40

40(39).
  • The jaw arcuate, with a vertical central keel ending in a median projection on the lower edge ... Zonitidae
  • The jaw not as in Zonitidae and Limacidae... 41

41(40).
  • The umbilicus small ... Euconulidae
  • The umbilicus large and wide ... Discidae

42(28).
  • The shell operculate; freshwater aquatic; the individuals either male or female (not hermaphrodite); breathing via a single gill attached within the mantle cavity; Prosobranchia ... Neritidae
  • The shell inoperculate; terrestrial; hermaphrodite; breathing air directly via the lung-like mantle cavity; Pulmonata... 43

43(42).
  • The shell thick-lipped; the shell relatively thick and opaque; the jaw not as in Zonitidae and Limacidae ... 44
  • The shell thin-lipped; the shell thin and translucent; the jaw arcuate, with a vertical central keel ending in a median projection on the lower edge ... Zonitidae

44(43).
  • The stalk of the bursa copulatrix with a diverticulum; ; @pÐml> ÅÄ ... Helicidae
  • The stalk of the bursa copulatrix without a diverticulum; @pÐml> ÅÄ; ‘A  ... Bradybaenidae

45(28).
  • The stalk of the bursa copulatrix with a diverticulum; ; @pÐml> ÅÄ ... Helicidae
  • The stalk of the bursa copulatrix without a diverticulum; @pÐml> ÅÄ; ‘A  ... Bradybaenidae

46(1).
  • The animal sinistral, with the genital orifice and anus opening on its left side, although the vestigial spire of the shell is slightly twisted to the right ... Ancylidae
  • The animal dextral, with the genital orifice and anus on its right side, although the vestigial spire of the shell inclines to the left ... Acroloxidae

47(1).
  • The mantle exhibiting a large dorsal island, demarcated by a conspicuous groove; the tail dorsally sharply keeled, the keel reaching to the mantle; with a small, totally internal, symmetrical shell ... 48
  • The mantle with a fingerprint-like pattern of fine, concentric rings; the tail dorsally bluntly keeled, but the keel falling short of the mantle; with a small, totally internal, asymmetrical shell... 49
  • The mantle with neither a dorsal island demarcated by a groove, nor a fingerprint-like pattern of folds; the tail not dorsally keeled; with a degenerate internal shell ... Arionidae

48(47).
  • relatively small, slender and almost worm-like when fully extended ... Boettgerillidae
  • not slender and worm-like when extended ... Milacidae

49(47).
  • The mantle with the concentric rings centred on its mid-line ... Limacidae
  • The mantle with the concentric rings centred to the right of the mid-line, above the respiratory pore ... Agriolimacidae

50(1).
  • The shell bent, with one margin quite sharply incurved and the other strongly curved outwardly ... Dreissenidae
  • The shell oblong, with the margin opposite the hinge more or less straight or only slightly incurved... 51
  • The shell not bent, the margin opposite the hinge outwardly curved... 52

51(50).
  • The shells dull black, the interior pearly white or pinkish ... Margaritiferidae
  • The shells brown, bright or dark greenish or greenish brown, glossy ... Unionidae

52(50).
  • The shell up to 3.5 mm in its maximum dimension ... Sphaeriidae
  • The shell 3.5 to 10 mm in its maximum dimension ... Sphaeriidae
  • The shell 10 to 20 mm in its maximum dimension ... Sphaeriidae
  • The shell 20 to 50 mm in its maximum dimension ... Sphaeriidae
  • The shell 50 to 85 mm in its maximum dimension ... Unionidae
  • The shell 85 to 150 mm in its maximum dimension ... Unionidae
  • The shell 150 mm in its maximum dimension or more ... Unionidae

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 non-marine molluscs of Britain and Ireland (slugs, snails and mussels). Version: 9th April 2015. delta-intkey.com’.

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