Fam. 17.—Choristidae. Shell spiral; four cephalic tentacles; eyes absent; two pedal appendages. Choristes.
Fam. 18.—Assimineidae. Eyes at free extremities of tentacles. Assiminea, estuarine, British.
Fam. 19.—Truncatellidae. Snout very long, bilobed; foot short. Truncatella.
| Fig. 30.—Valvata cristata, Müll. |
| o, Mouth. op, Operculum. br, Ctenidium (branchial plume). x, Filiform appendage (? rudimentary ctenidium). |
| The freely projecting ctenidium of typical form not having its axis fused to the roof of the branchial chamber is the notable character of this genus. |
Fam. 20.—Hydrobiidae. Shell with prominent spire; penis distant from right tentacle, generally appendiculated; brackish water or fluviatile. Hydrobia, British. Baikalia, from Lake Baikal. Pomatiopsis. Bithynella. Lithoglyphus. Spekia, viviparous, from Lake Tanganyika. Tanganyicia. Limnotrochus, from Lake Tanganyika. Chytra. Littorinida. Bithynia, British, fluviatile. Stenothyra.
Fam. 21.—Melaniidae. Spire of shell somewhat elongated; mantle-border fringed; viviparous; fluviatile. Melania. Faunus. Paludomus. Melanopsis. Nassopsis. Bythoceras, from Lake Tanganyika.
Fam. 22.—Typhobiidae. Foot wide; shell turriculated, with carinated whorls, the carinae tuberculated or spiny. Typhobia. Bathanalia, from Lake Tanganyika.
Fam. 23.—Pleuroceridae. Like Melaniidae, but mantle-border not fringed and reproduction oviparous. Pleurocera. Anculotus.
Fam. 24.—Pseudomelaniidae. All extinct.
Fam. 25.—Subulitidae. All extinct.