[M.D. Holloway]

The vipers are generally considered to be the most advanced family of snakes since they possess a very sophisticated venom delivery system. Large tubular fangs are placed in the front of the mouth and they are hinged, allowing them to be folded back when not in use. Their heads are covered with numerous small scales and their eyes have vertically eliptical pupils. All the vipers found in North America are in the subfamily of pit vipers (Crotalinae) having a pair of heat sensing pits located between each eye and nostril. The rattlesnakes are a truly American family of pit vipers since they are not found in the Old World and all but two species are found in the U.S. or Mexico. They are divided between two genera based on their head scales. Members of the genus Crotalus have numerous small scales on their heads while members of the genus Sistrurus have large scales (plates) on their heads.
Agkistrodon contortrix Copperhead
Agkistrodon piscivorus Cottonmouth
Crotalus adamanteus Eastern Diamondback Rattlesnake
Crotalus atrox Western Diamondback Rattlesnake
Crotalus cerastes Sidewinder
Crotalus enyo Lower California rattlesnake
Crotalus horridus Timber Rattlesnake
Crotalus lepidus Rock Rattlesnake
Crotalus mitchellii Speckled Rattlesnake
Crotalus molossus Blacktail Rattlesnake
Crotalus pricei Twin-spotted Rattlesnake
Crotalus ruber Red Diamond Rattlesnake
Crotalus scutulatus Mojave Rattlesnake
Crotalus tigris Tiger Rattlesnake
Crotalus viridis Western Rattlesnake
Crotalus willardi Ridgenose Rattlesnake
Sistrurus catenatus Massasauga
Sistrurus miliarius Pigmy Rattlesnake