Rattlesnake

Rattlesnake, any of two genera (Crotalus and Sistrurus) of pit vipers of the Americas, referring to a rattle, composed of successive pieces of sloughed-off dead skin, at the end of the tail. Rattlers have moveable fangs that fold up into the roof of the mouth when not in use and are shed and replaced every three weeks. They are extremely venomous snakes, with the diamondback rattler being the largest and most dangerous.