ON THE THEORY OF THE VERTEBRATE SKULL $47 



from the basisphenoid. The latter has the same relation to the 

 sella turcica in the bird as in the mammal ; and only differs from it 

 in that singular beak-like process, into which its inferior portion is 

 prolonged anteriorly, and which is produced, according to Kolliker,^ 

 by the coalescence with the basisphenoid of a distinct ossification^ 

 which is developed in the presphenoidal cartilage and partially repre- 

 sents the presphenoid of the mammal. The rest of the presphe- 

 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 cranio- 

 facial axis takes place in birds ; while nothing is more constant 

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

 to other parts, irrespectively of the manner in which ossification 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 con- 

 siderable 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, posterior 

 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 



^ ' Berichte von der Koniglichen Zool. Anstalt zu Wiirzburg,' 1849, p. 40. 



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