132 



STRUCTURE OF THE VERTEBRATES 



orbits, or eye sockets; and (4) the nasals, which roof the nostrils 

 or anterior nares. 



On the side of the face are: (1) the squamosal, in contact with 

 the parietal and occipital bones; and forming the temporal re- 

 gion and the bulla of the ear, and a part of the zygomatic arch, 

 to which jaw muscles are attached; (2) the jugal which com- 

 pletes the arch below the orbit; (3) the maxilla, a large bone 

 of the face, carrying most of the teeth and extending across the 

 roof of the mouth to form the hard palate; (4) the premaxilla, 

 which completes the skull anteriorly, bounds the ventral margin 



Maxilla 



Palatine^ 



Sphenoid 



Fig. 69. Opossum Skull, Ventral 

 View. 



of the nostrils, and bears. the front teeth; and (5) the lacrimal, 

 a small bone at the anterior margin of the orbit which is punc- 

 tured by the duct from the lacrimal gland. 



The ventral side of the skull shows few dermal bones except 

 in the anterior region. The roof of the mammalian mouth is bony, 

 and covered by the maxillae and the more posterior palatine 

 bones. The formation of a hard palate has carried the internal 

 opening of the nares back to the naso-pharynx. This is found 

 only in the mammals, the mammal-like reptiles, and a few other 

 reptiles. In the amphibia and most reptiles the nares open di- 

 rectly into the mouth cavity. 



