xii PHYLUM CHORDATA 499 



which curves outwards, then downwards, and finally forwards, 

 to unite with the fugal in the formation of the zygomatic arch. 

 Below the root of the process is a hollow, the glenoid fossa. 



Between the occipital and parietal bones, below and 

 behind the squamosal, are the tympanic and periotic bones. 

 The tympanic forms the bony part of the wall of the 

 external auditory meatus ; below it is dilated to form a 

 process (ty. but) projecting on the under surface of the 

 skull, the bulla tympani. The periotic (/. of) is a bone 

 of irregular shape enclosing the parts of the membranous 

 labyrinth of the internal ear ; externally it presents two 

 small openings, the fenestra ovalis and fenestra rotunda, 

 visible only when the tympanic is removed. The periotic 

 and tympanic are ankylosed together, but are loosely con- 

 nected with the surrounding bones. 



Roofing over the olfactory cavities are two fiat bones, 

 the nasals (nas). In front of the nasals are the pre- maxilla 

 — large bones which form the anterior part of the snout, 

 bear the upper incisor teeth, and give off three processes. 

 The maxilla {max), which form the greater part of the 

 upper jaw and bear the premolar and molar teeth, are 

 large, irregularly shaped bones, the upper surfaces of which 

 are spongy. They give off internally horizontal processes, 

 the palatine processes, which unite to form the anterior part 

 of the bony palate. A strong process which is given off 

 from the outer face of each maxilla and turns outwards and 

 then backwards to unite with the zygomatic process of the 

 squamosal and thus complete the zygomatic arch, is a 

 separate bone in the young, the malar or jugal (ju). 



The rest of the narrow bony palate, forming the roof of 

 the mouth and the floor of the nasal cavities, is formed by 

 the palatine plates of the palatine bones. The pterygoids 

 are small irregular bones, each of which articulates with 



