TIIK SKULLS OF MAMMALIA. 



273 



tegmen tijmpani. The second, narrower, elongated, and prismatic, 

 fits into the transverse channel behind the wedge-shaped pro- 

 cess {a). It corresponds with the pars mastoidea, and its rough 

 outer extremity appears on the exterior of the skull, between 

 the squamosal and ex-occipital. 



The internal division, convex and rounded below, is formed 

 by the pro-otic and opisthotic, and presents a large promontory 

 with the fenestra rotunda (f.o) on its posterior surface, while the 

 fenestra ovalis and Fallopian canal are visible upon its exterior. 

 The tympanic bone (Fig. 109, Ty) is large, and scroll-like in 

 form, very thick internally and below, and thin above and 

 externally, where it presents the aperture of the external audi- 

 tory meatus. It is by this thin upper and outer edge only, that 

 it eventually anchyloses with the tegmen tympani and pars 

 mastoidea, and hence, as its substance is very dense and brittle, 

 readily breaks off. 



In the adult Whale the tegmen tympani and pars mastoidea 



Fig. 109. 



V/7 





Fig. 109. — "Ear bones" of the adult Bakena anstralis. The upper figure gives the view 



from within ; the lower, from without. 



