GEOGRAPHICAL DISTRIBUTION 263 



by a complete or broken yellow line. The bite of the 

 Puff-adder is much dreaded. (2) Bitis gabonica, the River 

 Jack Viper, sometimes also called Puff-adder, is a large 

 and wonderfully coloured species, with a pair of coarse, 

 horn-like scales in contact with one another on the snout. 

 Sir H. Johnston describes its colours in life as " a carpet 

 pattern of alternate black, greenish-yellow, mauve, and 

 buff," the black patches being in the mid-dorsal line and 

 shaped like an hour-glass. It has been found in West 

 and East Africa and in Zanzibar. It is said to be sluggish , 

 but a bite from its enormous fangs is regarded as fatal. 

 According to Leonard Rogers its venom produces intra- 

 vascular clotting at first, and subsequently destroys the 

 coagulability of the blood. (3) Bitis nasicornis is another 

 large species with " horns " on the snout, but the horns 

 are separated from one another by small scales, and there 

 is more than one pair : the colouring is much like that 

 of B. gabonica, but the dark "hour-glass" patches in 

 the mid-dorsal line are completely divided in the middle, 

 and the light patches which separate them are sharply 

 nicked at either end ; on the head there is a large, dark 

 " spear-head " mark. B. nasicornis occurs in West Africa. 



(c) Cerastes (fig. 104, /). This genus resembles Echis in 

 having the lateral scales much smaller than the dorsal 

 scales, directed downwards, and furnished with serrated 

 keels ; but differs from Echis in the form of the ventral 

 shields, which have a sharp ridge at either side (fig. 103), 

 and in having two rows of small, hardly differentiated 

 sub-caudal shields. There are two species, and they are 

 coloured to harmonize with the desert in which they live. 

 Cerastes cornutus has a pair of "horns" above the orbits, 

 and is found in North-east Africa (and also in Arabia). 

 Cerastes vipera has no supra-orbital horns and occurs in 

 North-east Africa. 



(d) Echis (see p. 259). Both the Asiatic species of this 

 desert genus extend into Northern Africa, Echis carinatns 

 ranging almost to the Atlantic coast. 



(e) Athens. In this genus of tree-vipers the tail is long 



