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.
We now come to the bivalve shells with leaf-like gills. The Unionidæ contain two genera, Unio and Anodonta, commonly called freshwater mussels.
Unio tumidus.—Shell ovate, very solid, dark brown; common. See accompanying illustration, which shows the fringed branchial siphon which draws in food-bearing water, and the smaller anal siphon by which it gets rid of undigested matter.