638 OPHIDIA 



CHAP. 



attached. For further details see Fig. 180. The poison-fangs 

 are perforated, having a wide hole on the anterior side at the base, 

 in connexion with the large poison-gland ; the hole leads into a 

 canal, which opens gradually as a semi-canal on the anterior 

 surface of the distal third or quarter of the tooth. As usual in 

 poisonous snakes, several reserve-teeth are stowed away behind 

 the acting fang. When the latter is broken off or has served its 

 time it is cast off at the base, and the next reserve tooth takes 

 its place. The supply of reserve-teeth is indefinite, half-finished 

 teeth down to mere germs constantly growing. 



All the Viperidae are very poisonous, and all, except the 

 African Atractaspis, are viviparous. They include terrestrial, 

 arboreal, semi -aquatic, and burrowing types. The family is 

 cosmopolitan, excepting Madagascar and the whole of the 

 Australian region; it is divided into Vipers and Pit- Vipers. 



Sub-Fam. 1. Viperinae (Vipers). There is no sensory 



VIPERINAE. 



FIG. 171. Map showing the distribution of the Sub-Family Viperinae. Corsica and 

 Sardinia should be black in the map. 



external pit between the eye and the nose, and the maxillary is 

 not hollowed out above. The Vipers are absolutely restricted to 

 the Old World, ranging over the whole of Europe, Africa, and 

 Asia, with the exception of Madagascar ; their northern extension 

 is limited only by the permanently frozen condition of the under- 

 ground. Nine genera with about forty species are known. 



Causus with a few species in Africa and Azemiops feae in 

 Upper Burrnah are the only vipers which have the head covered 



