The advanced snakes of the families Colubridae, Elapidae, Hydrophiidae and Viperidae are a relatively recent development in geological time and are not found before the Oligocene. All the families are more or less widely distributed, except for the absence of specific families in particular areas and for a northern limit imposed by the permafrost, which precludes hibernation underground. Two thirds of living snakes are colubrids and the great majority of these are harmless to Man, since they are either without poison fangs (aglyphous) or have grooved fangs at the back of the mouth (opisthoglyphous). An exception is the boomslang (Dispholidus) of Africa, a back-fanged snake which can be dangerous.
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