388 ZOOLOGY 



(condyle) instead of two as in mammals. In the snakes and 

 lizards the quadrate bone is movable thus permitting great in- 

 crease in the caliber of the throat (Fig. 194). The cranium is 

 completely ossified thus showing an advance over the Amphibia. 

 The radius and ulna and the tibia and fibula are separate. 

 Vestiges of the pelvic girdles are found in some snakes, although 

 the limbs are wanting. 



„-a» 



Pig. 194. Skull of Rattlesnake (^Crotalus horridus). From Nicholson, after Huxley, ar, 

 articular portion of lower jaw; de, dentary portion; bo, basi-occipital; mx, maxilla, bearing poison 

 fang; na, nasal; pi, palatine, the front end being represented by a dotted line as though seen through 

 the maxilla; pmx, premaxilla; po, post frontal; pr, prefrontal; pt, pterygoid; gu, quadrate; sq, squa- 

 mosal ; tr, transverse bone. 



Questions on the figure. — Which bones bear teeth? Which are cranial and 

 which facial bones? What bones do you find common to the snake and the fish 

 (Pig. 180)? How do they differ in the two forms? What is the function of the 

 quadrate? How does it differ in the different groups of Vertebrates? 



418. Respiration. — Functional gills never occur, though gill- 

 slits are partly developed in the embryo only to close again 

 before hatching. The trachea is elongated and is supported by 

 cartilaginous rings as in the higher forms. It divides into two 

 bronchi, each of which passes to a spindle-shaped sac — the lung 

 — which is much simpler in its internal structure than those of 

 birds and mammals. In the snakes one lung (the left) is much 



