Mr. T. II. Huxley on the Theory of the Vei^tebrate Skull, 419 



ostrich's skull, traverses the bone at a much lower pomt, dividmg 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 

 comes 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 

 craniofacial axis, is in reality composed of three bones, an anterior 

 (petrosal), a posterior (mastoid), and a third, which is distinct from 

 the petrosal and mastoid in the chick, but is anchylosed with them in 

 the ostrich, and which has as yet received no name. I shall term 

 it, from its position with respect to the organ of hearing, the epiotic 

 bone, " OS epioticum*." 



The homology of the bone here called petrosal, with that of the 

 mammal, is admitted by all anatomists. The bone which lies imme- 

 diately in front of the petrosal is, with a no less fortunate unanimity, 

 admitted to be the homologue of the mammalian alisphenoid. But it 

 is worthy of particular remark, in reference to the shifting of the 

 relative positions of the lateral elements of the cranial wall, which 

 has been imagined to take place in the Ovipara, in consequence of the 

 supposed invariable disappearance of the squamosal from the interior 

 of their skulls ; that although precisely the same bones are visible 

 on the inner surface of the cranial cavity in the ostrich as in the 

 sheep, the squamosal being absent in both, yet in the ostrich the 

 third division of the trigeminal does not pass through the middle of 

 the alisphenoid, but between it and the petrosal. 



The orbitosphenoids appear like mere processes of the presphenoid, 

 and their relation to the optic nerves is altered in the same way 

 (when compared with the corresponding bones in the sheep) as that 

 of the alisphenoids to the trigeminal, that is to say, the nerves pass 

 behind, and not through them. 



The superior series of bones in the cranial wall is exactly the 

 same as in the sheep, and the parietals are distinct in the young 

 ostrich, as in the lamb. 



Attached to the exterior of the skull of the ostrich are, as in 

 the sheep, several bones ; but the appearance of some of these is 

 widely different from that of the parts which correspond with them 



* My reasons for considering this osseous element to be distinct from the 

 supraoccipital will be given below. 



27* 



