Large. Long tail has two raised keels. Hindfeet webbed. Eyes and valved nostrils on top of head. Adult dull olive with cream belly. Hatchling has black markings and straw-yellow belly. 20 - 80 eggs laid in hole in sandbank. Parent protects nest, hatchlings. Young eat insects, frogs. Adult ambushes mammals, birds, fish.
Almost extinct outside reserves. South of the Sahara.