266 



ON THE STRUCTURE OF THE SKULL. 



The superior and middle turbinal bones are greatly flattened 

 from side to side, and unite below and internally with the lamina 



Fig. 105. 



Fig. 105. — Longitudinal and vertical section of the skull of a Seal (Phoca vitulina). 



The premaxilla is absent. 



perpendicular is, or proper ethmoid, so that all direct communi- 

 cation with the superior and middle meatuses of the nose is shut 

 off below. The inferior turbinal, on the other hand, is exceed- 

 ingly large and complex in its structure. The orbito-sphenoids 

 are large and, ascending upon the front wall of the skull, unite 

 anteriorly behind and below the cribriform plate, so as to hide 

 nearly the half of the ethmoid when the base of the skull is 

 regarded from above. The presphenoid is relatively small. 



Less than half the length of the frontal bones enters into 

 the upper wall of the cranial cavity, the rest being devoted to 

 the roof of the nasal chambers. This part of the frontals is very 

 much narrower than the other, and is bent down at the sides, so 

 as to form two broad thin plates, which wall in the superior and 

 middle spongy bones, articulate below with the vomer and with 

 the palatine, and take the place of the os planum. 



The lower edge of the parietal unites with the front part of 

 the alisphenoid and with the ex-occipital, leaving a great infero- 

 lateral space, which is filled up in front and above by the 

 squamosal, and behind and below by the periotic. The squa- 

 mosal is relatively a small bone, but the periotic and the 

 tympanic, which are anchylosed with it, are very large. A 

 swollen pars mastoidea appears on the exterior of the skull, and 

 is hollowed internally by a cavity which opens into the cranium, 



