534 GEOGRAPHICAL ZOOLOGY. [part iv. 



Family 4— ARCADE. (6 Genera, 360 Species.) 



DiSTKiBUTiON. — The Arcada? are universally distributed, and 

 are most abundant in warm seas. The genus Leda is, however, 

 abundant in Arctic and Temperate regions, and Solenclla 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.— TRIGONIAD^. (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 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 Ethcria, 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.— CHAMIDiE. (1 Genus, 50 Species.) 



Distribution. — The Chamidae, 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 formation,:;. 



Family 8.— HIPPURITID^. (5 Genera, 103 Species.) 

 Fossils of doubtful affinity, from the Chalk formation. 



