418 Royal Society : — 



noidal cartilage is more or less completely ossified, and appears to be 

 represented in the ostrich by that part of the " vertical bony plate " 

 which lies behind the curved ridge referred to above ; while that 

 part of the plate which is situated in front of the ridge, answers to 

 the lamina perpendicularis of the ethmoid. 



Nothing can be more variable, in fact, than the mode in which 

 the ossification of the presphenoidal and ethmoidal portions of the 

 craniofacial axis takes place in birds ; while nothing is more con- 

 stant than the general form preserved by these regions, and their 

 relation to other parts, irrespectively of the manner in which ossifi- 

 cation takes place in them. And in these respects birds do but 

 typify the rest of the oviparous Vertebrata. 



If we compare the inferolateral walls of the ostrich's cranium 

 with those of the sheep, we find the most singular correspondences. 

 Posteriorly are the exoccipitals, which contribute to form the single 

 condyloid head for articulation with the atlas, but otherwise present 

 no important differences. In front of the exoccipital lies a consi- 

 derable bony mass, which unites, internally and inferiorly, with the 

 basioccipital and basisphenoid bones, and posteriorly is confluent 

 with the exoccipitals. Its anterior margin is distinguishable into 

 two portions, a superior and an inferior, which meet at an obtuse 

 angle. The anterior inferior portion articulates with the alisphenoid ; 

 the anterior superior portion with the parietal. The anterior, pos- 

 terior, and inferior relations of this bone are therefore the same as 

 those of the petromastoid of the sheep. 



Superiorly and posteriorly, a well-marked groove (which, however, 

 is not a suture) appears to indicate the line of demarcation between 

 the supraoccipital and this bone, whose pointed upper extremity 

 appears consequently to be wedged in between the supraoccipital and 

 the parietal. 



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

 tion and the exoccipital ; the third division of the trigeminal 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 mesencephalon. 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. 2), 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 Idnger a real suture (whatever it may have been), 

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



