SKULL 



133 



parts of the jaw in the lower Vertebrates: these become ossi- 

 fied and enclosed within the tympanic cavity situated within the 

 tympanic bone externally to the periotic. They thus represent 



FIG. 94. A, SKULL OF DELPHIXUS. (From M. Weber, after Boas.) B, SKULL 

 OF F<ETAL Balcena japonica. (From M. Weber, after Eschricht.) 



C, occipital condyle ; Fr, frontal ; Ju, jugal ; L, lacrymal : MX, maxilla ; n, 

 external nostril ; Na, nasal ; oe, exoccipital ; Os, supraoccipital ; Pa, parietal ; 

 Pal, palatine ; Pt, pterygoid ; Px, premaxilla ; Sq, squamosal ; Ty, tym- 

 panic and bulla tympani. 



the articular and quadrate, and are known as the malleus and incus 

 respectively. Having undergone a change of function, they form, 

 together with a third element the usually stirrup-shaped stapes, 

 a connected and articulated chain of auditory ossicles extending 



