Members of three orders of mammals—the Sirenia, the Carnivora, and the Cetacea—are adapted in varying degrees to life in fresh, brackish and salt water. Of these the sirenians, or sea-cows are probably the most retiring, the least aggressive, and the most local in their distribution. At least one of the Recent sirenians (Rhytina) was restricted to Bering Sea, while other living types are found in warmer waters, some in fresh water and others in brackish or salt water. Among the living sirenians, the dugong occurs in the coastal waters of the Philippines, Malaysia, the East Indies, Australia, and coasts bordering the Indian Ocean, including East Africa. The manatee is found not only in the warmer waters of the Atlantic Ocean, but also in the larger fresh-water rivers flowing into it, particularly the Amazon. The pinnipeds, which are carnivores and are generally known as seals, walruses, and sea lions, congregate in colonies at favorable localities, such as rocky islets; and at least one member of this group, the fur seal, makes long periodic migrations in the eastern Pacific from its summer breeding places in Bering Sea to its winter feeding grounds off California. Some of the living genera are more or less restricted to Arctic and Antarctic seas, while one genus (Monachus) is found only in subtropical waters. Hair seals (Phoca) closely related to marine types, are found in the fresh-water land-locked lakes of Finland, and in Lake Baikal in eastern Siberia. Another form of Phoca is an inhabitant...