Family 5.—TRIGONIADÆ. (1 Genus, 3 Species.)
Distribution.—The living Trigoniæ are confined to Australia, but there are 5 other genera fossil, containing about 150 species, and found in various formations from the Chalk to the Lower Silurian.
Family 6.—UNIONIDÆ. (7 Genera, 549 Species.)
Distribution.—The Unionidæ, or Fresh-water Mussels, are found in all the fresh waters of the globe, but some of the genera are restricted. Castalia, Mycetopus and Mulleria are confined to the rivers of South America; Anodon, to the Nearctic and Palæarctic regions; Iridina, and Etheria, to the rivers of Africa; Unio has a universal distribution, but is especially abundant in North America. About 60 fossil species are found in the Tertiary and Wealden formations.
Family 7.—CHAMIDÆ. (1 Genus, 50 Species.)
Distribution.—The Chamidæ, or Giant Clams, are confined to Tropical seas, chiefly among coral reefs. There are two other genera and 62 species fossil, ranging from the Chalk to the Oolite formations.
Family 8.—HIPPURITIDÆ. (5 Genera, 103 Species.)
Fossils of doubtful affinity, from the Chalk formation.
Family 9.—TRIDACNIDÆ. (1 Genus, 8 Species.)
Distribution.—The Tridacnidæ, or Clam-shells, are of very large size, and are confined to the Tropical regions of the Indian and Pacific Oceans. A few species have been found fossil in the Miocene formation.