THE SKULLS OF MAMMALIA. 267 



and extends under the anterior and posterior vertical semi- 

 circular canals. 



The tympanic forms a very thick bulla, prolonged externally 

 into an auditory meatus. It is firmly anchylosed with the pro- 

 otic regions of the periotic and with the squamosal, but for the 

 rest of its extent it is only applied to, and not anchylosed with, 

 the periotic. It is pierced by the carotid canal. 



The anchylosed squamosal, periotic, and tympanic are very 

 easily detached from the walls of the skull, as is the premaxilla 

 from the upper jaw. 



The skull of the Dugong (Halicore, Fig. 106) presents the 

 peculiarities of the cranial conformation of Mammals of the 

 order Sirenia in a very marked form. The basi-cranial axis is 

 almost flat above, but very thick. The suture between the 

 basi-occipital and the basi-sphenoid persists, but that between 

 the basi-sphenoid and the presphenoid is completely obliterated, 

 as is that between the presphenoid and the ethmoid, which last 



Fig. 106. 



Pmx 



Fig. 106. — Longitudinal and vertical section of the skull of a Dugong (Halicore Indicus). 



has the form of a stout bony plate, with an almost vertical 

 posterior edge, or crista galli. The upper median part of the 

 frontals is very narrow from before backwards, so that they cover 

 not more than the posterior half of the upper edge of the 

 ethmoid, and appear but very little on the roof of the cranial 

 cavity ; laterally and below, they are much expanded, and 

 produced forwards and outwards. The greater part of the roof 

 of the skull is furnished by the parietals, the longest antero- 



