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534 GEOGRAPHICAL ZOOLOGY. [part iv.
Family 4—AECAD^. (6 Genera, 360 Species.)
DiSTKiBUTiON.—The Arcadee are uuiversally distributed, and
are most abundant in warm seas. The genus Leda is, however,
abundant in Arctic and Temperate regions, and Solenella is con-
fined to the South Temperate zone. There are near 1,200 fossil
species, found in all strata as low as the Lower Silurian.
Family 5.—TEIGONIAD^. (1 Genus, 3 Species.)
Distribution.—The living Trigonice 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.—UNIONID^. (7 Genera, 549 Species.)
Distribution.—The Unionidae, or Fresh-water Mussels, are
found in all the fresh waters of the globe, but some of the genera
are restricted. Castalia, Mycetopus and Mulkria are confined to
the rivers of South America ;
Anodon, to the Nearctic and Palae-
arctic 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.—CHAMID^. (1 Genus, 50 Species.)
Distribution.—The Chamidse, 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.—HIPPUEITID^. (5 Genera, 103 Species.)
Fossils of doubtful affinity, from the Chalk formation.
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