164 



ZOOLOGY. 



the 

 ap- 

 we 

 the 

 to 



6. The Mammalian Skull. Figs. 85 and 86 represent, 

 in the most diagrammatic way, the main changes in /orra of 



the essential con- 

 stituents of 

 cranio-facial 

 paratus, as 

 pass from 

 amphibian 

 the mammalian 

 skull. The stu- 

 dent should com- 

 pare with the 

 more accurate 

 figures of both 

 skulls, and con- 

 vince himself that 

 he appreciates 

 the diagrammatic 

 rendering of these 

 parts. Now all 

 the distinctive 

 differences in 

 form of the dog's skull from the frog's are reducible to 

 two primary causes 



(1) The brain is enormously larger, and the brain-case is 



vastly inflated, so that 



(a) the otic capsule becomes embedded in the 



brain-case wall ; 



(b) the palato-pterygoid rod lies completely under- 



neath the brain-case instead of laterally 

 to it; 



(c) the squamosal tilts down and in, instead of 



down and out, and the lower jaw articulates 

 with its outer surface instead of below its 

 inner, and, moreover, with the enormous 

 distention of the brain-case it comes about 

 that the squamosal is incorporated in its 

 wall. 



(2) The maxilla anteriorly and the palatine posteriorly 



send down palatine plates that grow in to form the 



Fig. 85. SKULLS OF FROG AND Doo : Diagrammatic 

 side views. 



The shading does not mean the same as in figs. 81 and 83. 



