Fig. 



16, 17. A. oblongus, Lightfoot, {Patella.) — A. and P. lacustris. 



More lacustrine than No. 14, Mc. Oblong, apex turned to 



right. 



Planorbis. In fresh-water. 



18. P. corneus, Linn. {Helix.) — P. and H. cornu-arietis, purpureas, 



and similis, P. nana (young). — Ponds and ditches generally, C. 



19. P. albus, Milller. — Helix and P. hispidus, hirsutus. — Distribu- 



tion general, C. Finely ciliated in lines. 



20. P. glaber, Jeffreys. — P. lavis, Alder. — Penzance, Falmouth, Swan- 



sea, Belfast, etc., Mr. 



21. P. nautileus, Linn. {Helix.) — P. imhricatus, Midler; P. cristatus, 



Turbo nautileus, etc., Mc. With horny ridges. 



22. P. contort us, Linn. {Helix.) — P.and H. crassa, umbilicata. — Distri- 



bution general, C. Numerous close whorls ; aperture semi-lunar. 



23. P. carinatus, Miiller. — Helix Planorbis, Linn. ; P. planata and 



lutescens. — Generally distr., C. "Whorls more rapidly increased 

 than No. 24 ; keel more distinct. 



24. P. marginatum, Hrap. — H. Planorbis, H, P. limbata, complanata, 



Draparualdi turgidus, rhombens, etc. — Distr. gen., C. Whorls 

 more rhomboidal than 23. 



25. P. Vortex, Linn. {Helix.) — PI. 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, Miiller. — Helix and P.fontanus, complanatus, and len- 



ticularis. — Most parts of Brit., Me. 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. 



