XV. IO 



SNAKES 



4i3 



lungs is usually reduced, often to a rudiment, as in some limbless 

 lizards, and the other paired viscera tend to lie at different levels on 

 the two sides. The heart usually lies a quarter to a third of the way 

 down the body, and the carotid arches are asymmetrical, the right 

 common carotid artery tending to be suppressed. 



max 



temp ant 



Fig. 237. Skull of rattle-snake (Crotalus) with jaws partly and fully opened. 



Lettering as Fig. 214; sph-pt. the protractor-pterygoid muscle, which pulls the pterygoid 

 forward, causing it to push the ectopterygoid, which rotates the maxilla and erects the 

 fang; di. the digastric muscle that assists in opening the jaw; temp. ant. the anterior temporal 

 which shuts the mouth. The diagrams at the right show the actions of the levers that erect 

 the fang; g is the groove characteristic of crotaline snakes. (Modified after Gadow.) 



The snakes show nearly as much adaptive radiation as the lizards, 

 though there is less structural variation among them. The more pri- 

 mitive forms, with pelvic rudiments, include a number of small bur- 

 rowers such as Typhlops, as well as the large boas and pythons of the 

 family Boidae, which tend to be arboreal and amphibious in habits 

 and kill their prey by constriction. In general, the pythons lay eggs, 

 whereas the boas are ovoviviparous. 



The majority of living snakes belong to the family Colubridae, which 

 contains many medium-sized harmless snakes such as the grass-snake 

 (Natrix) and some moderately poisonous ones with grooved fangs at 



