602 Comparative Morphology of Chordates 



subcutaneous muscle, whereas in the anapsid skull the muscles which 

 elevate the lower jaw are inside the bony temporal arcade. 



In the several orders of mammals there is much variation in the 

 relation of the orbital cavity to the temporal space. There may be 

 no complete bony wall between these (as in most insectivores, rodents, 

 carnivores (Figs. 436B, 465 A), and some hoofed mammals), or they 

 may be more or less completely separated by a bony wall formed by 

 the joining of a downward-projecting process of the frontal bone and 

 an upward-projecting process from the zygomatic arch (as in the 

 horse, cattle, sheep, monkeys, apes, and man: Fig. 468). 



In possession of a secondary ("false") palate, because of which 

 the nasal passages are prolonged posteriorly (Fig. 435), the mammals 

 differ from birds, and from all modern reptiles except the crocodilians. 

 whose secondary palate makes it possible for air to pass directly from 

 the nasal cavities into the trachea without entering the mouth which 

 may, at the time, be filled with water. 



One of the most distinctive features of the mammalian skull is the 

 joint between upper and lower jaw. Whereas in all other bony 

 vertebrates the joint is between two cartilage bones, quadrate above 

 and articular below, in mammals it is formed by dermal bones — the 

 squamosal above and, below, a bone which is regarded as a much- 

 enlarged dentary. This dentary constitutes all of one half, or ramus, 

 of the lower jaw, but, in the reptilian lower jaw, each ramus includes 

 several separate dermal bones (see Table 1, p. 122). In contrast to 

 lizards, snakes, and birds, whose upper jawbones are to greater or less 

 extent movably articulated, all of the bones of the mammalian upper 

 jaw are immovably joined to one another. 



The absence of a small perforation in the roof of the skull between 

 the parietal bones (parietal foramen: p. I 19) is significant in that it 

 means that the mammal does not possess the "third eye" (pineal or 

 parietal eye), a problematic organ characteristic of many reptiles, both 

 past and present (Sphenodon and lizards). 



Mammalian Profiles. The main cranial cavity lodges the brain. 

 Among the several orders of mammals, there are great differences in 

 the size, relative to size of body, and form of the brain. These differ- 

 ences are necessarily reflected in the proportions of the skull and, 

 again, in the external form of the head. Also, the region in front of 

 the brain-case is subject to great variation. In some mammals the 

 nasal region is greatly elongated. In others it is relatively short. Com- 

 paring such "lower" mammals as anteaters, rats, and mice with 

 carnivores and monkeys (Fig. 466), there is to be seen in the latter a 

 marked shortening of the jaws and nasal region. This difference is a 

 reduction with reference to length of body and quite irrespective of the 



