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Family 9.—TRIDACNID^E. (1 Genus, 8 Species.)
Distribution.—The Tridacnidse, 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.—CARDIAD^E. (1 Genus, 200 Species.)
Distribution.—The Cardiadae, 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.-LUCINID.E. (8 Genera, 178 Species.)
Distribution.—The Lucinidae 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 Leplon, to the Atlantic. There are nearly 500 extinct species, ranging from the Tertiary back to the Silurian formation.
Family 12.—CYCLADID^E. (3 Genera, 176 Species.)
Distribution.—The Cycladidae 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.—CYPRINID.E. (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—VENERID.E, (10 Genera, 600 Species.)
Distribution.—Universal. Zucinopsis 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.—MACTRID^E. (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.—TELLINIDJE. (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.—SOLENID.E. (3 Genera, 63 Species.)
Distribution.—All Temperate and Tropical seas. There are 80 fossil species which range back to the Carboniferous epoch.
Family 18—MYACHLE. (6 Genera, 121 Species.)
Distribution.—All seas. Pa-nopcea 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.—ANATINID^E. (8 Genera, 246 Species.)
Distribution.—All seas. Fholadomya is from Tropical Africa; Myadora from the Western Pacific; Myochama and Chamostnra are Australian. There are about 400 fossil species, ranging back to the Lower Silurian formation.
Family 20.—GASTKOCILENID^ (5 Genera, 40 Species.)
Distribution.—Temperate and warm seas. Aspergittum ranges from the Red Sea to New Zealand. There are 35 fossil species, ranging back to the Lower Oolite.
Family 21.—PHOLADID^E. (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 many of them only to the Eocene. In striking contrast we have the vast antiquity of most of the families of MoU lusca, as shown in the following table of their range taken from Mr. Woodward's work, but re-arranged, and somewhat modified.
Nor is this enormous antiquity confined to family types alone. Many genera are equally ancient. The genus Lirvfula lias existed from the earliest Palaeozoic times down to the present day ; while Terebratula, Rhynelwnella, Discina, Nautilus, Natica, Pleurotomaria, Patella, Dentalium, Mytilus, and many other living forms, range back to the Palaeozoic epoch. That groups of such immense antiquity, and having power to resist such vast changes of external conditions as they must have been subject to, should now be widely distributed, is no more than might reasonably be expected. It is only in the case of sub-genera and species, that we can expect the influence of recent geological or climatal changes to be manifest; and it must be left to special students to work out the details of their distribution, with reference to the general principles found to obtain among the more highly organised animals.