Inland Seas are seas entirely surrounded by land (see Caspian Sea, Dead Sea, and, for general discussion, Lake). Enclosed Seas have communication with the ocean restricted to one opening, which may take the form of one, two, or more straits close to each other. The best known are the White Sea of the Arctic Ocean; the Baltic, Zuyder Zee, Hudson’s Bay, Gulf of Mexico, and Mediterranean, with the Adriatic and Black Sea, of the Atlantic; the Red Sea and Persian Gulf of the Indian Ocean; and the Yellow Sea and Sea of Okhotsk of the Pacific. They are all cut off from general oceanic circulation and very largely from tides, but the result is not stagnation. The Baltic and Black Sea are but slightly saline on account of the number of large rivers falling into them, and the fresh surface-water flows out as a regular current, liable indeed to be checked, and even reversed for a time, but in the main persistent; while the salt water flows in uniformly as an undercurrent. A state of equilibrium is arrived at, so that periodical fluctuations of salinity do not affect the average of a number of years. The water of the Mediterranean and Red Sea is much salter than that of the ocean, which therefore flows in as a surface-current, while the dense very salt water escapes below. In the case of the Baltic and Black Sea dilution by rivers, in that of the Mediterranean and Red Sea concentration by evaporation maintains a circu- lation. Winds and differences of barometric pressure are, as in inland seas, great factors in producing variable currents. (See Baltic Sea, Black Sea, Mediterranean Sea, Red Sea, &c.) Partially Enclosed Seas may be (a ) comparatively shallow irregular channels through which strong tides sweep, or (b ) ocean basins cut off by barriers barely rising to the surface, or remaining permanently submerged, in which case there may be no break of continuity in the ocean surface to indicate the sea. Seas of the first description are related to shallow enclosed seas, but are much affected by tides and ocean currents; the principal are the Kara Sea of the Arctic Ocean, Baffin Bay and North Sea of the Atlantic, Behring Sea and Japan Sea of the Pacific. They are subject to considerable temperature changes owing to their proximity to land. Seas coming under the second category combine the peculiarities of the open ocean and of deep inland seas. The Caribbean Sea of the Atlantic, the China Sea, Java Sea, and numerous small seas of the eastern archipelago of the Pacific are the best examples. Their chief peculiarity is that the temperature of the water instead of falling uniformly to the bottom becomes stationary at some intermediate position corresponding to the top of the barrier. They are usually very deep. (See North Sea, Norwegian Sea, and Pacific Ocean.) Other Seas.— Coral Sea, Arabian Sea, Sea of Bengal, are names, now dropping out of use, to designate parts of the ocean. “Sargasso Sea” is an expression devoid of geographical meaning (see Atlantic Ocean, vol. iii. p. 20).

Firths and Estuaries.— A river entering the sea by a short estuary flows over the surface, freshening it to a considerable extent, and, if the force of its current is not too great, the rising tide slowly forces a wedge of sea water up between river and river bed, withdrawing it rapidly when ebb sets in. In a firth that is large compared with the river falling into it, judging from results recently obtained in the Firth of Forth, a state of equilibrium is arrived at, the water increasing in salinity more and more gradually as it proceeds seawards, the disturbing influence of the tide becoming less and less, and the vertical distribution of salinity more and more uniform until the river water meets the sea, diffused through a nearly homogeneous mass with a density little inferior to that of the ocean. Between the extreme cases there are numerous gradations of estuary depending on the ratio of river to sea inlet.

Deposits.— All seas within about 300 miles of continental land, whatever may be their depth, are paved with terrigenous débris, and all at a greater distance from shore are carpeted with true pelagic deposits (see Pacific Ocean). Marine Fauna and Flora.— The mixing of river with sea water produces a marked difference in the fauna and flora of seas. Where low salinity prevails diatoms abound, probably on account of the greater amount of silica dissolved in river water, and they form food for minute pelagic animals and larvae, which are in turn preyed upon by larger creatures. In some seas, such as the North Sea, there are many celebrated fishing beds on the shallow banks of which innumerable invertebrate animals live and form an inexhaustible food-supply for edible fishes. Naturalists have remarked that in temperate seas enormous shoals of relatively few species are met with, while in tropical seas species are very numerous and individuals comparatively few. Organisms, such as the corals, which secrete carbonate of lime appear to flourish more luxuriantly in warmer and salter seas than in those which are colder and fresher.

The geological and dynamic aspects of seas are treated of in Geology (vol. x. p. 284 sq.) and Geography (Physical); and in Atlantic Ocean, Baltic Sea, Black Sea, Indian Ocean, Mediterranean Sea, North Sea, Norwegian Sea, Pacific Ocean, Polar Regions, and Red Sea the general geographical and physical characters of oceans and seas are described. In Meteorology some account is given of the influence of the sea on climate, and chemical problems connected with the ocean are discussed in Sea Water.