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CHAP. XXII.] MOLLUSCA. 533
• known
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
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.