THE SKULLS OF MAMMALIA. 217 



it contains the posterior vertical semicircular canal. Between 

 the hinder edge of this division and the ex-occipital, the eighth 

 pair of nerves leaves the skull. 



The inferior, or internal, edge of the periotic bone in the Bird, 

 and that of the pars petrosa in the Beaver, comes into relation 

 with the basi-occipital and basi-sphenoid ; externally, each 

 exhibits the fenestra ovalis and rotunda, and is related, above, 

 to the squamosal. 



In fact, the only noteworthy differences between the ornithic 

 periotic, and the Mammalian pars petrosa and mastoidea, are that 

 the former becomes anchylosed with the adjacent bones of the 

 cranial wall, while the latter remain separate from them ; and 

 that, while the periotic articulates, above, with the parietal in 

 the Bird, the corresponding ossification in the Mammal is 

 separated from that bone by the squamosal. 



On the former distinction it would of course be absurd to lay 

 any weight ; as regards the latter, it is deprived of all signi- 

 ficance by the circumstances that in some Birds — as, e.g., the 

 common Fowl — the squamosal interposes between the periotic 

 and the parietal in the wall of the skull ; and that in some Mam- 

 mals — as, e.g., the Sheep — the squamosal is completely excluded 

 from the skull, and the pars petrosa unites with the parietal. 



The simple anatomical comparison of the parts appears, then, 

 to be amply sufficient to demonstrate, that the pars petrosa and 

 mastoidea of the Beaver correspond in every essential respect 

 with the periotic mass of the Bird, and therefore with the pro-otic, 

 opisthotic, and epiotic bones of Keptiles and Fishes. On the 

 other hand, no one has ever doubted that the petrosal and mas- 

 toid of the Beaver answer to the petrosal and mastoid of Man ; and 

 therefore we are led by the comparison of adult structure, merely, 

 to exactly the same conclusion as that at which we arrived by the 

 study of development, to wit, that the pars petrosa and pars mas- 

 toidea of Man answer to the periotic bones of the low r er Vertebrates. 



In front of the periotic, the side w r all of the cranium is formed 

 by an alisphenoid, anchylosed below with the basi-sphenoid ; and, 

 still more anteriorly, by a large orbito-sphenoid, united inferior! y 

 with the presphenoid, which is distinct from the basi-sphenoid 

 behind, and from the ethmoid in front. 



