Limnæa (Omphiscola) glabra.—Also amphibious. About the same size as truncatula. Local. Inner lip rather thick and reflected on the base of the penultimate whorl.
Limnæa (Ancylus) fluviatilis.—“Freshwater limpet.” Shell, rather limpet-like, with a hooked apex (whence its generic name), adheres to stones or piles in running water. Common. I once dredged a large water-beetle with three of these shells adhering to its wing-cases; thus it would be transported to fresh habitats.
Limnæa (Acroloxus) lacustris.—Like the former but more local, and preferring sluggish or still waters. Shell more oblong, thinner, and apex twisted to the left instead of to the right as in fluviatilis.
The sub-order Pectinibranchiata (comb-like gill) contains the genera Neritina, Paludina, and Valvata, in all of which there are two tentacles with eye at the base, and an operculum to the shell.
Neritina fluviatilis.—Solid, glossy, chequered brown, white, and purple (but also a lemon-coloured variety). Operculum semi-lunar, orange, with a projection which serves as a lock to keep the operculum in position. Not rare in England; on stones in running water. See illustration below, which also shows above L. (Ancylus) fluviatilis.
Paludina (Vivipara) contecta.—Shell dark green with darker bands. Conical. Suture very deep. Operculum horny. Viviparous. Local.
Paludina (Vivipara) vivipara.—More common than contecta. Shell more oval, not so glossy, light greenish yellow, suture not so deep, no umbilicus, apex blunt.
Neritina and Ancylus.
Paludina (Bythinia) tentaculata.—(The eyes in this genus are not on foot-stalks; the operculum is shelly instead of horny). Common in slow water and ditches. Shell semi-transparent, yellowish, mouth oval, angulated above. Operculum made of plates rising one above another formed at different stages of growth.