416 REPTILES xv. 10- 



vvarning and perhaps prevents the snake from being trodden on by 

 large mammals. 



Most of the Viperidae are highly poisonous, though the bite of the 

 European adder {Vipera berus) is seldom fatal to man. The venom is 

 predominantly haemolytic in action. The fangs are canalized, the 

 canal having apparently being evolved by the progressive deepening 

 of a groove until its margins have come into apposition. The fact 

 that the fangs are erected when the snake strikes and can be folded 

 back along the roof of the mouth when not in use, makes it possible 



Fig. 240. Drawings of three frilled lizards (Chlamydosaurus) and a Grammatophora 



(at right) to show the bipedal habit. (Drawings made by Heilmann from photographs 



of the lizards running at full speed, taken by Saville Kent.) 



for these structures to be very long, about 1 inch in the case of a large 

 puff adder. 



Some of the American pit-vipers are very large, the dreaded bush- 

 master (Lachesis) reaching about 10 ft. The majority of the Viperidae 

 bear their young alive and the finding of late embryos within the 

 bodies of female adders and rattle-snakes may have given rise to the 

 tale and that these reptiles temporarily hide their young by swallowing 

 them in the face of danger. 



1 1 . Superorder Archosauria 



We have seen that about 130 million years ago the diapsid stock 

 produced the most successful modern reptile group, the Squamata 

 (Fig. 221). Much earlier an even more successful type had developed 

 from the Eosuchia, having as its outstanding feature the habit of walk- 

 ing on the hind legs. Creatures of this type were the dominant land 

 animals of the later Mesozoic, and they include the dinosaurs and 

 pterosaurs. Crocodiles are the only living descendants of the group 



