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.
[Illustration: _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.
_Paludina (Bythinia) leachii._--Much smaller and less common than the last. Distinct umbilicus; mouth almost circular.
_Paludina (Paludestrina) ventrosa._--A brackish-water shell, swarming where found, _e.g._ from Erith to Gravesend, and in East Anglia. Shell small, thin, semi-transparent.
_Paludina (Paludestrina) jenkinsi._--A larger shell, not confined to brackish water and spreading very rapidly. Swarms where found. A variety has a marked keel which sometimes bears bunches of spines at equal distances.
_Paludina (Paludestrina) stagnalis._--Larger and with more whorls. Not so common.
_Paludina (Pseudamnicola) anatina._--Small, sub-conical, deep suture. Found in brackish water, and apparently identical with _Hydrobia_ or _Paludestrina similis_, which I used to find by the Thames, where it is now apparently extinct.
_Valvata piscinalis._--Globular, suture very deep, circular mouth, operculum concentrically spiral. In ponds and slow water. Shell yellowish, but commonly covered with conferva.
_Valvata cristata._--Much smaller; shell disk-shaped. Frequents the roots of flags. Shell striated and more or less ridged, but the name _cristata_ refers to the plume-like appearance of its breathing apparatus.