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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 f. 



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 v/all 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 

 in the mammal. This is least the case with the largest and upper- 

 most of these bones, which lies upon the parietal above, the ali- 

 sphenoid in front, and the exoccipital behind ; while internally it is 

 in relation with the petromastoid. 



This bone lies immediately above an articular surface, which is 

 furnished to the os quadratum by the petrosal, and more remotely 



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

 supraoccipital will be given below, 

 t See Note II. 



