A large, herbivorous aquatic mammal of the order Sirenia that contains two Recent families: Dugongidae for the genera Dugong (dugong, one species) and Hydrodamalis (Steller's sea cow, one species hunted to extinction); and Trichechidae for the single genus Trichechus (manatees, three species). The dugong inhabits coastal regions in the tropical parts of the Old World, and some individuals go into estuaries and rivers. Steller's sea cow inhabited the Bering Sea, and was the only Recent member of this order adapted to cold waters. Manatees live along the coast and in coastal rivers in the southeastern United States, Central America, the West Indies, northern South America, and western Africa.