HAP. xxiJ.] MOLLUSCA. 533 



Class.— CONCHIFERA; 



The Coucliifera, or ordinary Bivalve Molluscs, may Le 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.— OSTREID^. (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/E. (3 Genera, 94 Species.) 



Distribution,— The Aviculida?, 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 Mytilidre, or Mussels, have a world-wide 

 distribution, There is one fresh-water species, M'hich inhabits 

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

 the Carboniferous epoch. 



