REPTILIA 387 



(condyle) instead of two as in mammals. The lower jaw articu- 

 lates indirectly, by means of the quadrate bone, with the skull. 

 This gives a very movable jaw and in the snakes especially, 

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

 more compactly fused and completely ossified than among the 

 Amphibia. The ulna and radius and the tibia and fibula are not 

 fused as in the frog. Rudiments of the pelvic girdles are found 

 in some snakes, although the limbs are wanting. 



FIG. 194. Skull of Rattlesnake (Crotalus durissus). From Nicholson, after Huxley, or, 

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

 fang; no, 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; qu, 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 

 (Fig. 1 80)? 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 lobings than those of birds and 

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



