Family 11.—TURRITELLIDÆ. (5 Genera, 230 Species.)
Distribution.—Universal. Cæcum is found in north temperate seas only. The other genera are mostly tropical, but some species reach Iceland and Greenland. There are near 300 species fossil, ranging from the Neocomian to the Pliocene formations.
Family 12.—LITTORINIDÆ. (9 Genera, 310 Species.)
Distribution.—The Littorinidæ are mostly found on the coasts in shallow water; as the common Periwinkle (Littorina littorea). They are of world-wide distribution; but Solarium and Phorus are tropical; while Lacuna, Skenea, and most species of Rissoa are Northern. About 180 species are fossil, ranging from the Permian to the Pliocene formations.
Family 13.—PALUDINIDÆ. (4 Genera, 217 Species.)
Distribution.—The Paludinidæ, or River-snails, are all fresh-water, and range over the whole world. Paludina (60 sp.), is confined to the Northern Hemisphere; Ampullaria (136 sp.), is tropical; Amphibola (3 sp.), inhabits New Zealand and the Pacific Islands; Valvata (18 sp.), North America and Britain. There are 72 fossil species of Paludina and Valvata, in the Wealden formation and more recent fresh-water deposits.
Family 14.—NERITIDÆ. (10 Genera, 320 Species.)
Distribution.—All warm seas, ranging north to Norway and the Caspian Sea. Neritina and Navicella inhabit fresh or brackish waters, the latter confined to the countries bordering the Indian Ocean and the islands of the Pacific. There are 80 fossil species, from the Trias, Lias, and Eocene formations down to recent deposits.
Family 15.—TURBINIDÆ. (10 Genera, 425 Species).
Distribution.—The genus Trochus (200 sp.) has a world-wide range, but the other genera are mostly tropical, and are most abundant in the Indian and Pacific Oceans. There are more than 900 fossil species, found in all parts of the world, from the Lower Silurian to the Tertiary formations.