The approximately 2,700 snake species, of which about four fifths are nonvenomous, are distributed throughout the temperate and tropical zones of the world (except in New Zealand, Ireland, and some isolated oceanic islands) and are found in greatest profusion in the tropics. About two thirds of all snake species belong to the family Colubridae; most of these are nonvenomous. Among the harmless colubrid snakes of North America are the garter snakes (including the ribbon snake), the water snakes, the green, or grass, snakes, the black snakes, the racers, the king snakes (including the milk snake), and the bull, hognose, and rat snakes. The family Boidae (boas and pythons) includes the world's largest snakes, the South American anaconda and the Asian reticulated python, as well as the smaller boa constrictor and the tree and sand boas.
Most poisonous New World snakes belong to the pit viper family; these include the copperhead, water moccasin, rattlesnake, fer-de-lance, and bushmaster. Venomous Old World snakes are the true vipers, including the adder and the asp, and members of the cobra family, including the mamba of Africa and the krait of Asia. The poisonous coral snakes of the New World also belong to this family. The venomous sea snakes inhabit tropical oceans.