Fam. 7.—Planaxidae. Shell with pointed spire; a short pallial siphon. Planaxis.
Fam. 8.—Cyclostomatidae. Pallial cavity transformed into a lung; pedal centres concentrated; a deep pedal groove. Cyclostoma, shell turbinated, operculum calcareous, British. Omphalotropis.
Fam. 9.—Aciculidae. Pallial cavity transformed into a lung; operculum horny; shell narrow and elongated. Acicula.
Fam. 10.—Valvatidae. Ctenidium bipectinate, free; hermaphrodite; fluviatile. Valvata, British.
Fam. 11.—Rissoidae. Epipodial filaments present; one or two pallial tentacles. Rissoa. Rissoina. Stiva.
Fam. 12.—Litiopidae. An epipodium bearing three pairs of tentacles and an operculigerous lobe with two appendages; inhabitants of the Sargasso weed. Litiopa.
Fam. 13.—Adeorbiidae. Mantle with two posterior appendages; ctenidium large and capable of protrusion from pallial cavity. Adeorbis, British.
Fam. 14.—Jeffreysiidae. Head with two long labial palps; shell ovoid; operculum horny, semicircular, carinated. Jeffreysia.
Fam. 15.—Homalogyridae. Shell flattened; no cephalic tentacles. Homalogyra, British. Ammoniceras.
Fam. 16.—Skeneidae. Shell depressed, with rounded aperture; cephalic tentacles long. Skenea, British.