Fam. 18. Assimineidae.—Ctenidium replaced by a pulmonary sac, no true tentacles, eye-peduncles long, retractile; radula that of Hydrobia; shell small, conoidal, operculum corneous, nucleus sub-lateral. Eocene——. Genera: Assiminea, Acmella.
Fam. 19. Skeneidae.—Radula resembling that of Hydrobia; shell very small, depressed, widely umbilicated, operculum corneous. Pleistocene——. Single genus, Skenea.
Fam. 20. Jeffreysiidae.—Mantle with two pointed ciliated appendages in front, tentacles ciliated, eyes sessile, far behind the base of the tentacles; marginal teeth sometimes absent; shell small, thin, pellucid, whorls rather swollen, operculum with marginal nucleus, divided by a rib on the inner face. Recent. Genera: Jeffreysia, Dardania. Marine, living on algae.
Fam. 21. Litiopidae.—Epipodium with cirrhi on each side, operculigerous lobe with appendages; radula rissoidan; shell small, conical, columella truncated, operculum corneous. Eocene——. Genera: Litiopa, living on the Sargasso weed, suspended by a long filament; Alaba, Diala.
Fam. 22. Adeorbidae.—Radula essentially rissoidan; shell depressed, circular or auriform, widely umbilicated, operculum corneous, paucispiral, nucleus excentrical. Pliocene ——. Principal genera: Adeorbis, Stenotis, Megalomphalus.
Fam. 23. Viviparidae.—Snout blunt, tentacles long, right tentacle in the male deformed, pierced with a hole corresponding to the aperture of the penis, two cervical lobes, the right being siphonal, foot with an anterior transverse groove; teeth broad, shallowly pectinate at the ends; shell turbinate, whorls more or less rounded, aperture continuous, operculum corneous, nucleus sub-lateral, with a false sub-central nucleus on the external face. Animal ovoviviparous. Fresh-water. Cretaceous ——. Genera: Vivipara (= Paludina), subg., Cleopatra, Melantho, Tulotoma; Tylopoma (Tertiary), and Lioplax.
Fam. 24. Valvatidae.—Branchia exserted, bipectinate, carried on the back of the neck, a filiform appendage (Fig. [66], p. 159) on the right of the neck, penis under the right tentacle, prominent, eyes sessile, behind the tentacles; radula like that of Vivipara; shell small, turbinate or flattened, operculum corneous, nucleus central. Fresh water. Jurassic ——. Single genus, Valvata.
Fam. 25. Ampullariidae.—Snout with two tentacles, tentacles proper very long, tapering, eyes prominently pedunculate, two cervical lobes, the left siphonal, respiratory cavity divided by a partition, a large branchia in the right chamber, the left functioning as a pulmonary sac (Fig. [65], p. 158); radula large, central tooth multicuspid, base broad, lateral and marginals falciform, simple or bicuspid; shell large, turbinate or flattened, spire small, whorls rounded; operculum generally corneous, nucleus sub-lateral, false nucleus as in Vivipara. Fresh water. Cretaceous ——. Single genus, Ampullaria (subg., Ceratodes, Pachylabra, Asolene, Lanistes, and Meladomus).
Fam. 26. Cerithiidae.—Branchial siphon present, short, eyes variable in position; central tooth small, evenly cusped, lateral hollowed at base, multicuspid, marginals narrow; shell long, turriculate, whorls many, generally tuberculate, varicose or spiny, aperture sometimes strongly channelled; operculum corneous, subcircular, nucleus nearly central. Marine or brackish water. Trias ——. Principal genera: Triforis, shell small, generally sinistral; Fastigiella, Cerithium (Fig. [12], p. 16), Bittium, Potamides (subg., Tympanotomus, Pyrazus, Pirenella, Telescopium, Cerithidea, Lampania, all brackish water), Diastoma (Eocene), Cerithiopsis; Ceritella (Jurassic), Brachytrema (Jurassic), and Planaxis (subg., Quoyia and Holcostoma).
Fam. 27. Modulidae.—No siphon, radula of Cerithium; shell with short spire, columella strongly toothed at the base, aperture nearly circular. Recent. Single genus, Modulus.