These snakes range in size from the diminutive hump-nosed viper , Hypnale hypnale , that grows to an average only 30-45 cm ( 12-18 in ) , to the bushmaster , Lachesis muta ; a species known to reach a maximum of 3.65 m ( 12.0 ft ) in length-the longest venomous snake in the New World .
Pit vipers range in size from the Hump-nosed Viper , Hypnale hypnale , which grows to the length of around 30-45 centimetres ( 12-18 in ) , to the South American Bushmaster , Lachesis muta , which is known to grow to a length of 3.65 metres ( 12.0 ft ) , making it the longest venomous snake in the New World .