392 



The par vagum passes out between the bony mass under descrip- 

 tion and the exoccipital ; the third division of the trigermnal leaves 

 the skull between it and the alisphenoid. The portio dura and the 

 portio mollis enter it by foramina very similarly disposed to those in 

 the sheep. Superiorly there is a fossa on the inner face of the bone, 

 which corresponds with a more shallow depression in the sheep, and, 

 like it, supports a lobe of the cerebellum. Finally, the anterior 

 inferior edge of the bone traverses the middle of the fossa which 

 receives the mesericephalon. In every relation of importance, there- 

 fore, this bony mass corresponds exactly with the petromastoid of 

 the sheep, while it differs from it only in its union with the exocci- 

 pitals and the supraoccipital posteriorly, and its contact with the 

 craniofacial axis below. 



If from the ostrich we turn to the young chick (fig. 3), the con- 

 dition of this part of the walls of the skull will be found to be still 

 more instructive. The general connexions of the corresponding bony 

 mass, Pt. M. Ep., are as in the ostrich; but while it is even more 

 evident that the groove appearing to separate its upper end from the 

 supraoccipital is no longer a real suture (whatever it may have been), 

 a most distinct and clear suture, of which no trace is visible in the 

 ostrich's skull, traverses the bone at a much lower point, dividing it 

 into an inferior larger piece, united with the exoccipital, and a supe- 

 rior portion, anchylosed with the supraoccipital. The latter contains 

 the upper portions of the superior and external semicircular canals. 



Moreover, on endeavouring to separate the inferior bone from the 

 exoccipital, it readily parts along a plane which traverses the fenestra 

 ovalis externally, and the anterior boundary of the foramen of exit 

 of the par vagum internally. The posterior smaller portion remains 

 firmly adherent to the exoccipital, while the other larger portion 

 conies away as a distinct bone. 



The latter answers exactly to the mammalian petrosal, while the 

 small posterior segment corresponds with the mammalian mastoid. 

 Like that of the mammal, it is eventually anchylosed with the 

 petrosal ; but unlike that of the mammal, it is also, and indeed at 

 an earlier period, confluent with the exoccipital*. 



Thus, to return to the ostrich's skull, the bony mass interposed 

 between the exoccipital, supraoccipital and parietal bones, and the 



* See Note I. 



