CHAP. XXII.] MOLLUSCA. 533 



Class.— CONCHIFERA. 



The Conchifera, or ordinary Bivalve Molluscs, may be distin- 

 guished from the Brachiopoda by having their shells laterally 

 unsymmetrical, while the valves are generally (but not always) 

 equal. They are mostly marine, but a few inhabit fresh water. 

 As the distribution of some of the families presents points of 

 interest, we shall treat them in the same manner as the marine 

 Gasteropoda. 



Family 1.-0STEEID^. (5 Genera, 426 Species.) 



Distribution. — The Ostreidse, including the Oysters and Scal- 

 lops, are found in all seas, Arctic as well as Tropical. There are 

 nearly 1,400 species fossil, ranging back to the Carboniferous 

 period. 



Family 2.— AVICULID^. (3 Genera, 94 Species.) 



Distribution, — The Aviculidse, or Wing-shells and Pearl 



Oysters, are characteristic of Tropical and warm seas, a few only 



ranging into temperate regions. Nearly 700 fossil species are 



• known from various formations ranging back to the Devonian, 



and Lower Silurian. 



Family 3.— MYTILID^. (3 Genera, 217 Species.) 



Distribution. — The Mytilidse, or Mussels, have a world-wide 

 distribution. There is one fresh-water species, which inhabits 

 the Volga. There are about 350 fossil species, ranging back to 

 the Carboniferous epoch. 



