Fig. 



16,17. A. OBLONGUS, Lightfoot, {Patella.) — A. aud P. lacnstris. 

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



riglit. 



Planorbis. In fresh-water. 



18. P. CORNEUS, Linn. {Helix.) — P. and H. cornii-arietis, purpurews, 



and similis, P. nana (j'oung). — Ponds and ditches generally, C. 



19. P. ALBUS, Mailer. — Helix and P. hispidus, hirsutus. — Distribu- 



tion general, C. Finely ciliated in lines. 



20. P. GLABER, Jeffreys. — P. lavis, Alder. — Penzance, Palmouth, Swan- 



sea, Belfast, etc., Mr. 



21. P. NAUTiLEUS, Linn. {Helix.) — P. imbricahis, Miiller; P. cristatus, 



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



22. P. C0NT0RTas,i2««. {Helix.) — P.and H. crassa, umiilicaia. — Distri- 



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

 2b. P. CARINATUS, Mailer. — Helix Planorbis, Linn. ; P. planata and 

 lufescens. — Generally distr., C. Whorls more rapidly increased 

 than No. 24 ; keel more distinct. 



24. P. MARGiNATUS, Drap. — H. Planorbis, H., P. limbata, complanata, 



Braparualdi tim/idas, rhonibeus, 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, Jft'^fe;-. — Helix Anii P. fontanus, complanatus, and len- 



ticularis. — Most parts of Brit., Mc. Very compressed. 



28. P. LACUSTRis, Lifjhlfoot. — P. nitidns, var., Segmentina lineaia, P. 



nautileus and claitsnlatiis. — Ponds and ditciies near London, etc., 

 Mc. With internal divisions ; lower disc flat. 



