Any of several large, cylindrical, herbivorous marine mammals of the order Sirenia, having a paddlelike tail and rounded front flippers, including the manatee, dugong, and the very large species Hydrodamalis stelleri of the northern Pacific that became extinct in the late 1700s. Also called sirenian. or Steller's sea cow Extinct aquatic mammal (Hydrodamalis gigas) that lived around islands in the Bering Sea. It was discovered in 1741 and described by a member of Vitus Bering's expedition. At least 24 ft (7.5 m) long, it had no teeth, a small head, and a broad, horizontal, forked tail fluke; its dark brown skin was sometimes streaked or spotted with white. It browsed on seaweed. Russian sealers hunted it for food and fur; by 1768 the entire population, estimated at about 5,000, had been exterminated. The term also refers to dugongs and manatees