The majority of venomous snakes in the United States are pit vipers, varieties that all have elliptical pupils and wide, triangular heads as common features. Indigenous non-venomous snakes are commonly identified by round pupils and oval heads, with the exception of the venomous coral snake. The coral snake is a dangerous species easily confused with harmless king or milk snakes, as they are similar in appearance except for slight variations in their color patterns.
Pit viper species, such as cottonmouths and rattlesnakes, are easy to identify based on their heads and pupils. The coral snakes are much more difficult to differentiate from harmless species. Coral snakes have red, black and yellow bands where the red is fringed by yellow on either side, whereas harmless tri-color snake's red bands are bordered by black. "Red touch yellow, kills a fellow, red touch black, venom lack," is a well-known mnemonic rhyme that was created to help people remember how to tell the difference.
Out of 3,000 known snake species in the world, more than 600 are venomous, and there is no rule of thumb about how to tell venomous from non-venomous that is consistently correct 100 percent of the time. The notion of identifying venomous snakes by elliptical pupils is a common misconception, but in addition to Coral Snakes some of the deadliest snakes in the world have round pupils, such as the mamba and cobra species.