SIRENIA ARE REMARKABLY adapted to a coastal, estuarine or riverine life. The order includes two families, the Dugongidae and the Trichechidae. The Dugongidae are represented by two subfamilies, each comprising a single species: the Dugonginae, which includes the dugong (Dugong dugon) and the Hydrodamalinae, which includes Steller's sea cow (Hydromalis gigas). The Hydrodamalinae has occasionally been treated as a family. Hydromalis is the largest member of the order, reaching a length of 10 m, but is believed to be extinct as there were at most only about 2 000 in the Bering Sea in 1741 when it was first discovered by Dr Wilhelm Steller, a ship's doctor. Fearless of humans, they soon fell prey to sealers and whalers. In 1963 the crew of a Russian whaling ship reported a group of marine creatures that were not whales or seals and which in size and other features matched the description of Steller's sea cow (Nishiwaki, 1972). There is thus the faint possibility that they are not extinct. The second family, the Trichechidae, is represented by three species: the Amazon manatee (Trichechus inunguis), the American manatee (T. manatus), and the African manatee (T. senegalensis), which occurs from Senegal to Angola.