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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 Aviculide, 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.
FAMILY 5.-TRIGONIADÆ. (1 Genus, 3 Species.) DISTRIBUTION.—The living Trigoniæ are confined to Australia, but there are 5 other genera fossil, containing about 150 species, and found in various formations from the Chalk to the Lower Silurian.
FAMILY 6.—UNIONIDÆ. (7 Genera, 549 Species.) DISTRIBUTION.- The Unionidæ, or Fresh-water Mussels, are found in all the fresh waters of the globe, but some of the genera are restricted. Castalia, Mycetopus and Mulleria are confined to the rivers of South America ; Anodon, to the Nearctic and Palæarctic regions; Iridina, and Etheria, to the rivers of Africa; Unio has a universal distribution, but is especially abundant in North America. About 60 fossil species are found in the Tertiary and Wealden formations.
FAMILY 7.-CHAMIDÆ. (1 Genus, 50 Species.) DISTRIBUTION.—The Chamidæ, or Giant Clams, are confined to Tropical seas, chiefly among coral reefs. There are two other genera and 62 species fossil, ranging from the Chalk to the Oolite formations.
FAMILY 8.-HIPPURITIDÆ. (5 Genera, 103 Species.) Fossils of doubtful affinity, from the Chalk formation.
FAMILY 9.-TRIDACNIDÆ. (1 Genus, 8 Species.) DISTRIBUTION.- The Tridacnidæ, or Clam-shells, are of very large size, and are confined to the Tropical regions of the Indian and Pacific Oceans. A few species have been found fossil in the Miocene formation.
FAMILY 10.-CARDIADÆ. (1 Genus, 200 Species.) DISTRIBUTION.—The Cardiadæ, or Cockles, are of world-wide distribution. Another genus is fossil, and nearly 400 fossil species are known, ranging back to the Upper Silurian formation.
FAMILY 11.-LUCINIDÆ. (8 Genera, 178 Species.) DISTRIBUTION.—The Lucinidæ inhabit the Tropical and Temperate seas of all parts of the world ; but the genus Corbis is confined to the Indian and Pacific Oceans, Montacuta and Lepton, to the Atlantic. There are nearly 500 extinct species, ranging from the Tertiary back to the Silurian formation.
FAMILY 12.-CYCLADIDÆ. (3 Genera, 176 Species.)
The Cycladidæ are small fresh- or brackishwater shells found all over the globe. The genus Cyclas is most abundant in the North Temperate zone, while Cyrena inhabits the warmer shores of the Atlantic and Pacific, but is absent from the West Coast of America. There are about 150 species fossil, ranging back from the Pliocene to the Wealden formations.
Family 13.-CYPRINIDÆ. (10 Genera, 176 Species).
DISTRIBUTION. --Universal. Cyprina and Astarte are Arctic and North Temperate ; Cardita is Tropical and South Temperate. There are several extinct genera and about 1,000 species found in all formations as far back as the Lower Silurian.
FAMILY 14.-VENERIDÆ, (10 Genera, 600 Species.)
DISTRIBUTION.—Universal. Lucinopsis is confined to the North Atlantic; Glauconeza to the mouths of rivers in the Oriental region; Meroe and Trigona to warm seas. There are about 350 fossil species, ranging back to the Oolitic period.
FAMILY 15.—MACTRIDÆ. (5 Genera, 147 Species.) DISTRIBUTION.-All seas, but more abundant in the Tropics. Gnathodon is found in the Gulf of Mexico; Anatinella in the Oriental region. There are about 60 fossil species, ranging back to the Carboniferous period.
Family 16.-TELLINIDÆ. (11 Genera, 560 Species.) DISTRIBUTION.-All seas; most abundant in the Tropics. Galatea is confined to African rivers. There are about 60 fossil species, mostly Tertiary, but ranging back to the Carboniferous period.
FAMILY 17.-SOLENIDÆ. (3 Genera, 63 Species.) DISTRIBUTION. -All Temperate and Tropical seas. There are 80 fossil species which range back to the Carboniferous epoch.
FAMILY 18.—MYACIDÆ. (6 Genera, 121 Species.) DISTRIBUTION.-All seas. Panopoa inhabits both North and South Temperate seas ; Glycimeris, Arctic seas. There are near 350 fossil species, ranging back to the Lower Oolite formation.
FAMILY 19.-ANATINIDÆ. (8 Genera, 246 Species.) DISTRIBUTION.—All seas. Pholadomya is from Tropical Africa; Myadora from the Western Pacific; Myochama and Chamostræa are Australian. There are about 400 fossil species, ranging back to the Lower Silurian forination.
FAMILY 20.–GASTROCHÆNIDÆ. (5 Genera, 40 Species.)
DISTRIBUTION.- Temperate and warm seas. Aspergillum ranges from the Red Sea to New Zealand. There are 35 fossil species, ranging back to the Lower Oolite.
FAMILY 21.-PHOLADIDÆ. (4 Genera, 81 Species.) DISTRIBUTION.—These burrowing molluscs inhabit all Temperate and warm seas from Norway to New Zealand. There are about 50 fossil species, ranging back to the epoch of the Lias.
General Remarks on the Distribution of the Marine Mollusca.
The marine Mollusca are remarkable for their usually wide distribution. About 48 of the families are cosmopolitan, ranging over both hemispheres, and in cold as well as warm seas. About 15 are restricted to the warmer seas of the globe ; but several of these extend from Norway to New Zealand, a distribution which may be called universal, and only 2 or 3 are absolutely confined to Tropical seas. Two small families only, are confined to the Pacific and Indian Oceans. Marine fishes, on the other hand, have a much less cosmopolitan character, no less than 30 families having a limited distribution, while 50 are universal. Some of these 30 families are confined to the Northern seas, some to the Atlantic and Mediterranean, and a considerable number to the Indian Ocean and Western Pacific. Many of these families, it is true, are much smaller than those of the Mollusca, which seem to possess very few of those small isolated families of two or three species only, which abound in all the Vertebrate classes. These differences are no doubt connected with the higher organisation of fishes, which renders them • more susceptible to changed conditions of life ; and this is indicated by the much less antiquity of existing families of fishes, the greater part of which do not date back beyond the Cretaceous epoch, and inany of them only to the Eocene. In striking con trast we have the vast antiquity of most of the families of Mol