ON THE THEORY OF THE VERTEBRATE SKULL 549 



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

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

 ately 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 

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

 most of these bones, which lies upon the parietal above, the alisphenoid 

 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 

 it helps to roof in the tympanic cavity but takes no share in the 

 formation of the fenestra ovalis. It sends a free pointed process 

 downwards and forwards, which does not articulate with the jugal. 

 Except in this particular, however, the bone in question resembles 

 in every essential relation the squamosal of the sheep, while to the 

 same extent it differs from the mastoid of that animal. 



I have stated that in the ostrich this bone does not appear upon 

 the inner surface of the wall of the skull, and in this respect, while 

 it resembles the squamosal of the sheep and Ruminants generally, it 

 differs from that of most other Mammalia, in which the squamosal 

 makes its appearance in the interior of the skull, between the parietal, 



1 See Note IL 



