The difference in density and chemical constitution of salt and fresh water draws the line between the marine and the fluviatile faunae; below a certain depth, probably not far from 120 fathoms, the absence of light and the increase of pressure would prove an insurmountable barrier to most of the class. Fishes are able to resist extreme cold, and to regain vitality after, having been apparently frozen, but the average of cold has an important influence on their geographical distribution; the average temperature of the water for the year has been usually taken as the regulator of this distribution, but Dana has shown that the line of temperature established by the average of the 30 coldest days in the year gives the clue to the limits of the marine faunae. A few arctic species are the same in America and Europe, migrating southward from the same northern centre; hut below this region the marine fauna of America is essentially tropical, and that of Europe essentially temperate. In the Atlantic the zones of temperature are remarkably modified by the Arctic, Gulf stream, and African currents; on the American side the temperate zone extends only from Cape Cod to Cape Hatteras, about 10 degrees of latitude, while on the eastern it extends from the Swedish coast to the Cape Verd islands, nearly five times as many degrees; while the tropical zone, which in America extends from Cape Hatteras to 25° S., or 00 degrees, on the other side embraces only about 20 degrees on the Guinea coast of Africa. As a few instances of local distribution, in contradistinction to the cosmopolitan scomberoids and cyprinoids, may be mentioned the American cottoids and goni-odonts, the Mediterranean sparoids, the tropical sciaenoids, squammipennoe, and mullets; the pleuronectidoe of the temperate regions; the tropical fresh-water characini of America and Africa; the true salmons of arctic and cold regions; and the marine labroids, and freshwater chromids.

Estimating the number of vertebrates at 20,000, the number of living species of fish may be set down at 10,000, of which more than 6,000 are described.-Of all the ver-tebrata, fishes are by far the most numerous and widely distributed in the earth's strata; their remains are found from the Silurian to the tertiary formations, and are of great aid in determining the changes of the surface of our planet during successive and long geological periods. The first great geological division, the primary age, comprises the lower and upper Silurian and the Devonian; till the close of this age there were no air-breathing animals, and in the Devonian period fishes were the lords of creation; the latter has, therefore, been very properly called the "age of fishes." Agassiz, in his Becherches sur lesp)ois-sons fossiles (1833-'43), laid the foundation of fossil ichthyology; 1,000 species are described in the most complete and scientific manner, with superb illustrations. He divides fossil fishes, as he afterward did the recent ones, into four orders, according to the form and structure of their scales; these orders, ganoids, placoids, ctenoids, and cycloids, have been sufficiently described in the article Comparative Anatomy (vol. v., p. 172). Three fourths of all known fossil fishes belong to the ctenoids and cycloids, which occur in all formations from the chalk upward; the remaining fourth belong chiefly to the ganoids (with enamelled scales like the garpike and sturgeon) and the placoids (like sharks and rays), and extend through all the fossiliferous strata, but are most numerous in the coal, Jurassic, chalk, and tertiary formations; no fish with ctenoid scales (like the perch) or cycloid (like the cod) is found below the chalk.

The forms of the earlier fishes were many of them very strange; the pectorals were very small and always in advance of the ven-trals; above the chalk, the ventrals begin to approach nearer the head; they were not so fully developed as our fishes, but seem to have been, like the sturgeon, arrested in their development. During this epoch the sea covered the greater part of the surface of the globe, and all animals whose remains have been preserved were without exception aquatic, breathing by gills; the climate must have been uniform and warm; the dry land had hardly appeared above the waters, and all creation was as silent as in mid ocean.-For the systematic classification of fishes, and the history of the science, see Ichthyology.