BELUGA (Delphinapterus leucas)  , also called the " white whale," a cetacean of the family Delphinidae, characterized by its rounded head and uniformly light colour . A native of the Arctic seas, it extends in the western Atlantic as far south as the river St Lawrence, which it ascends for a considerable distance . In colour it is almost pure white; the maximum length is about twelve feet; and the back-fin is replaced by a low ridge . Examples have been taken on the British coasts; and individuals have been kept for some time in captivity in America and in London . See CETACEA .