Family 10.—CARDIADÆ. (1 Genus, 200 Species.)
Distribution.—The Cardiadæ, or Cockles, are of world-wide distribution. Another genus is fossil, and nearly 400 fossil species are known, ranging back to the Upper Silurian formation.
Family 11.—LUCINIDÆ (8 Genera, 178 Species.)
Distribution.—The Lucinidæ inhabit the Tropical and Temperate seas of all parts of the world; but the genus Corbis is confined to the Indian and Pacific Oceans, Montacuta and Lepton, to the Atlantic. There are nearly 500 extinct species, ranging from the Tertiary back to the Silurian formation.
Family 12.—CYCLADIDÆ. (3 Genera, 176 Species.)
Distribution.—The Cycladidæ are small fresh- or brackish-water shells found all over the globe. The genus Cyclas is most abundant in the North Temperate zone, while Cyrena inhabits the warmer shores of the Atlantic and Pacific, but is absent from the West Coast of America. There are about 150 species fossil, ranging back from the Pliocene to the Wealden formations.
Family 13.—CYPRINIDÆ. (10 Genera, 176 Species).
Distribution.—Universal. Cyprina and Astarte are Arctic and North Temperate; Cardita is Tropical and South Temperate. There are several extinct genera and about 1,000 species found in all formations as far back as the Lower Silurian.
Family 14.—VENERIDÆ. (10 Genera, 600 Species.)
Distribution.—Universal. Lucinopsis is confined to the North Atlantic; Glauconeza to the months of rivers in the Oriental region; Meroe and Trigona to warm seas. There are about 350 fossil species, ranging back to the Oolitic period.