XV. IO 



SNAKES 



4i5 



the back of the maxillae; these are known as back- fang snakes or 

 opisthoglyphs. The South African boomslang (Dispholidus) is one of 

 the few whose bite may be lethal in man. Dasypeltis, the egg-eating 

 snake, is also a member of this group ; it swallows the eggs whole and 

 crushes them with special tooth-like processes of the neck vertebrae. 

 The family Elapidae contains the cobras, kraits, and coral snakes, 

 all highly poisonous with quite small and relatively non-movable fangs 

 at the front of the maxilla, and a venom predominantly neuro-toxic in 



<l'S tp 



Fig. 239. Head of crotaline snake (Lachesis) after removal of skin. 



d. duct of poison gland, bending at base of fang; dig. digastric muscle; g. sensory pit or 



groove ; n. nostril ; p.g. poison gland ; ta. and tp. anterior and posterior portions of temporalis 



muscle; tr. trachea. (From Gadow.) 



action. All the poisonous Australian snakes belong to this group. The 

 hood of the cobra is expanded by the long cervical ribs and probably 

 has a warning (sematic) function. The king cobra (Hamadryas) is the 

 largest poisonous snake and reaches 18 feet in length. 



The very poisonous sea-snakes (Hydrophiidae) are related to the 

 elapids. Their tails are vertically compressed for swimming; some 

 species can hardly move on land. Like many freshwater snakes they 

 are (with a few exceptions) ovoviviparous. 



The family Viperidae consists of the vipers of the Old World and 

 the rattle-snakes and pit-vipers, mainly from the New World. The 

 two latter groups, placed in the subfamily Crotalinae, are distinguished 

 by the presence of a remarkable sensory pit on each side of the head 

 between eye and nostril (Fig. 239). This is highly sensitive to tem- 

 perature changes and helps the snake to detect warm-blooded prey. 

 The rattle-snakes are, of course, also noted for their caudal append- 

 ages, which are composed of articulated rings and modified skin. One 

 ring is formed at each moult, though the older and most posterior 

 ones break off periodically. The rattle is vibrated voluntarily as a 



