534 
GEOGRAPHICAL ZOOLOGY. 
[part IV. 
Family 4. — ARC AD M. (6 Genera, 360 Species.) 
Distribution— The Arcadaa are universally 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. — TRIGONIADiE. (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. — IJNIONIDiE. (7 Genera, 549 Species.) 
Distribution. — The Unionidse, 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 Palse- 
arctic regions ; Iridina, and Mheria, 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 . — HIPPURITID^E. (5 Genera, 103 Species.) 
Fossils of doubtful affinity, from the Chalk formation. 
