THE SKULLS OF MAMMALIA. 



211 



metrical in the region of the nasal aperture, but elsewhere they 

 are pretty nearly symmetrical. But it is the nasal bones which 

 exhibit the greatest distortion, the left and right being very 

 unequal in size and dissimilar in form. 



The jugal and the lachrymal commonly become anchylosed. 



The basi-occipital, as in the Balwnoidea, gives off a lateral 

 downward process, which unites, behind (Fig. Ill), with an out- 



EO 



Pmi 



Ft 



Fig. 111. — Longitudinal and vertical section of the skull of the White Whale or Beluga 

 (Delphi napter us). * marks a distinct bony element interposed between the alisphenoid, 

 the parietal, and basi-cranial axis ; a, the ossified falx. 



ward prolongation of the ex-occipital, and, in front, with the 

 pterygoid, to constitute the inner wall of a deep chamber for the 

 tympano-periotic. But the roof of this chamber is chiefly 

 formed by the very large alisphenoid, which extends outwards 

 to unite with the frontals, parietals, and squamosals, and back- 

 wards to the ex-occipitals. The aperture which is left between 

 the hinder edge of the alisphenoid, the ex-occipital, basi-occipital, 

 and basi-sphenoid, is exceedingly small, so that the tympano- 

 periotic is still more shut out from the cranial cavity than in 

 Balsena. In Hijperoodon and Orca the aperture is still further 

 reduced ; but this peculiarity cannot be said to be a distinctive 

 character of the Delphinoid skull, as in Platanista the aperture 

 is large, and the periotic appears in the interior of the cranial 

 cavity in the ordinary way. 



