Class.—CONCHIFERA.
The Conchifera, or ordinary Bivalve Molluscs, may be distinguished 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.—OSTREIDÆ. (5 Genera, 426 Species.)
Distribution.—The Ostreidæ, including the Oysters and Scallops, 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.—AVICULIDÆ. (3 Genera, 94 Species.)
Distribution.—The Aviculidæ, 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.—MYTILIDÆ. (3 Genera, 217 Species.)
Distribution.—The Mytilidæ, 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.
Family 4.—ARCADÆ. (6 Genera, 360 Species.)
Distribution.—The Arcadæ are universally distributed, and are most abundant in warm seas. The genus Leda is, however, abundant in Arctic and Temperate regions, and Solenella is confined to the South Temperate zone. There are near 1,200 fossil species, found in all strata as low as the Lower Silurian.