REPTILIA. 383 



No sternum occurs in turtles and snakes. When present, as 

 in lizards and crocodiles, it is formed in connection with the 

 ventral end of the ribs. 



The skull articulates with the first vertebra by one surface 

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



pmx 



de~~ 



FIG. 191. Skull of Rattlesnake (Crotalus durissus). From Nicholson, after Huxley. 

 ar, articular portion of lower jaw; de, dentary portion; bo, basi-occrpital ; 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, pre- 

 f rental; pt, pterygoid; qu, quadrate; sq, squamosal; 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. 177)? 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 ? 



articulates indirectly, by means of the quadrate bone, with the 

 skull. This gives a very movable jaw and in the snakes espe- 

 cially, increases the caliber of the throat (Fig. 191). The 

 skull 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. 



