Fam. 26.—Nerineidae. All extinct.
Fam. 27.—Cerithiidae. Shell with numerous tuberculated whorls; aperture canaliculated anteriorly; short pallial siphon. Cerithium. Bittium. Potamides. Triforis. Laeocochlis. Cerithiopsis.
Fam. 28.—Modulidae. Shell with short spire; no siphon. Modulus.
Fam. 29.—Vermetidae. Animal fixed by the shell, the last whorls of which are not in contact with each other; foot small; two anterior pedal tentacles. Vermetus. Siliquaria.
Fam. 30.—Caecidae. Shell almost completely uncoiled, in one plane, with internal septa. Caecum, British.
Fam. 31.—Turritellidae. Shell very long; head large; foot broad. Turritella, British. Mesalia. Mathilda.
Fam. 32.—Struthiolariidae. Shell conical; aperture slightly canaliculated; siphon slightly developed. Struthiolaria.
Fam. 33.—Chenopodidae. Shell elongated; aperture expanded; siphon very short. Chenopus, British. Alaria, Spinigera, Diartema, extinct.
Fam. 34.—Strombidae. Foot narrow, compressed, without sole. Strombus. Pteroceras. Rostellaria. Terebellum.