391 



rated 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 

 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. 



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



