MOLLUSCA. 
533 
CHAP. XXII.] 
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. — OSTKEIDiE, (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. — AVICULIDiE. (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. — MYTILIDiE. (3 Genera, 217 Species.) 
Distribution. — The Mytilidee, 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. 
