Order Sirenia - Manatees and Dugongs
There are 4 living species of sirenians, 3 manatees and the dugong. A fifth species, Steller's sea cow, of the North Pacific and Bering Sea, was exterminated by overhunting in the 1700s. Sirenians, like cetaceans, are totally aquatic. They are the only herbivorous marine mammals. As a consequence, they tend to be less marine than members of other marine mammal groups. In fact, manatees spend much or all of their lives in fresh or brackish water. All 4 living species are restricted to a tropical and subtropical habitat; Steller's sea cow was unique; it inhabited cold temperate to subarctic waters.
Sirenians have the following morphological characteristics in common: robust body; tough, thick skin with little hair; two nostrils on top or at the front of a thick muzzle; no ear pinnae; no hind limbs; mammary nipples located near the axillae; forelimbs modified into flippers; horizontally flattened tail; and dense, swollen bones.