Illustrated Index of British Shells / Containing figures of all the recent species

by G. B. Sowerby

21. P. nautileus, Linn. (Helix.)—P. imbricatus, Müller; P. cristatus, Turbo nautileus, etc., Mc. With horny ridges.

22. P. contortus, Linn. (Helix.)—P.and H. crassa, umbilicata.—Distribution general, C. Numerous close whorls; aperture semi-lunar.

23. P. carinatus, Müller.—Helix Planorbis, Linn.; P. planata and lutescens.—Generally distr., C. Whorls more rapidly increased than No. 24; keel more distinct.

24. P. marginatus, Drap.—H. Planorbis, H., P. limbata, complanata, Draparnaldi turgidus, rhombeus, etc.—Distr. gen., C. Whorls more rhomboidal than 23.

25. P. Vortex, Linn. (Helix.)—Pl. compressa.—Distr. gen., C. Many whorls, very compressed.

26. P. spirorbis, Linn. (Helix.)—P. vortex, var.—Distr. general, C. Fewer whorls than No. 25; mouth not angular.

27. P. nitidus, Müller.—Helix and P. fontanus, complanatus, and lenticularis.—Most parts of Brit., Mc. Very compressed.

28. P. lacustris, Lightfoot.—P. nitidus, var., Segmentina lineata, P. nautileus and clausulatus.—Ponds and ditches near London, etc., Mc. With internal divisions; lower disc flat.