442 MANUAL OF ZOOLOGY SECT. 



of the plastron of the Chelonia are probably of a similar 

 character. 



In the skull ossification is much more complete than in 

 the Amphibia, the primary chondrocranium persisting 

 to a considerable extent only in some Lizards and in 

 Hatteria, and the number of bones is much greater. 



Ntt La, 



A* 



FIG. 251. A, lateral view of skull of Rattlesnake (CrotalUS). B. O. basi-occipital ; 

 B. S. basi-sphenoid ; E. O. exoccipital ; F. O. fenestra ovalis ; La. conjoined 

 lacrymal and prefrontal ; L.f. articulation between lacrymal and frontal ; Mn. 

 mandible ; MX. maxilla ; Nn. nasal ; PI. palatine ; Pmp. pre-maxilla ; P. Sf>h. 

 pre-sphenoid ; Pt. pterygoid ; Qu. quadrate ; Sg. squamosal ; //, V. foramina 

 of exit of the second and fifth cranial nerves. B, transverse section at point 

 lettered B in Fig. A ; T. trabeculae. (After Huxley.) 



The parasphenoid is reduced, and its place is taken by 

 large basioccipital, basisphenoid, and presphenoid. The 

 lower jaw articulates with the skull through the intermedia- 

 tion of a quadrate bone, which is movable in the Lizards 

 and Snakes, fixed in Hatteria, the Chelonia and Crocodilia. 



A remarkable feature of the skull of the Snakes (Fig. 251), 

 is the free articulations of the bones of the jaws, permitting of 



