The family Viperidae is a group of venomous front-fanged snakes widespread throughout world but absent from Madagascar and Australia. The maxilla is shortened and the attached single long fang has an enclosed canal for venom. The fangs are folded into the roof of the mouth but swivel forward during biting. There are four subfamilies: Azemiopinae (a single species, Azemiops feae), Causinae (night adders – a single genus of primitive vipers, Causus spp.), Crotalinae (pit vipers) and Vipirinae (true vipers).