THE LOWER VERTEBRATA. 



165 



bony palate, cutting off a nasal passage from the 

 mouth cavity, and carrying the posterior nares from 

 the front part of the mouth, as they are in the frog, 

 to the pharynx. Hence the vomer of the dog lies 

 not in the roof of the mouth, but in the floor of this 

 nasal passage. 



7. The Auditory Ossicles. The quadrate cartilage of 

 the frog is superseded by the squamosal as the suspensorium 

 of the lower jaw. It is greatly reduced, therefore ; but it 

 is not entirely absent. In the young mammal, a quadrate 

 cartilage can be traced, con- 

 nected with the palato- 

 pterygoid cartilage, and ar- 

 ticulating with Meckel's 

 cartilage. Its position is, 

 of course, beneath ' the 

 squamosal, and just outside 

 the otic capsule. As de- 

 velopment proceeds, the in- 

 crease in size of the quadrate 

 does not keep pace with that 

 of the rest of the skull 

 structures. It loses its con- 

 nexion with the palato- 

 pterygoid, and apparently 

 ossifies as a small ossicle the 

 incus of the middle ear. A 

 small nodule of cartilage, cut off from the proximal end of 

 Meckel's cartilage, becomes the malleus. Hence these 

 small bones seem to be the relics of the discarded jaw 

 suspensorium of the frog utilized in a new function. Con- 

 siderable doubt, however, attaches to this interpretation 

 doubt that, if anything, is gaining ground. The stapes 

 would appear to be derived from the hyoid arch and to 

 answer to the frog's columella. 



8. Comparison of Frog's Skull and Dog's. Besides 

 these great differences in form, there are important differ- 

 ences in the amount and distribution of centres of ossifica- 



Fig. 80. SKULLS OF FROG AND DOG : 

 Diagrammatic end views. 



Shading as in fig. 85. 



