Mr..T. H. Huxley on the Theory of the Vertebrate Skull 417 



In most adult birds, as is well known, the bones of the cranium 

 have coalesced so completely as to be undistinguishable. But in 

 the chick, and to a greater or less extent, in the adult struthious 

 bird, the boundaries of the various bones are obvious enough ; and 

 I will therefore select for comparison with the mammalian skull 

 that of an ostrich, and that of a young chicken. 



The craniofacial axis of the bird has the same general figure as 

 that of the sheep, consisting of a thick, solid, median portion, 

 lodging the sella turcica ; of a posterior, horizontally, and of an ante- 

 rior, vertically, expanded division ; but it is comparatively shorter and 

 thicker in correspondence with the greater shortness, in proportion 

 to its depth, of the cranial cavity. The sella turcica is very deep, and 

 its front wall is very thick. The lower and anterior half of this wall 

 is produced into a long tapering process, which extends forwards 

 far beyond the anterior limit of the bony lamina perpendicularis of 

 the ethmoid, to end in a point. 



Overlying this process, and articulated with more than the pos- 

 terior half of its upper surface, there is, in the ostrich, a strong, 

 thick, vertical, bony plate, narrower in front and behind than in the 

 middle, and below than above. A curved vertical ridge on 'each 

 lateral surface marks the line of its greatest transverse diameter, 

 and seems to indicate a primitive division of the mass into two 

 parts, an anterior and a posterior. The latter is connected above 

 with the bony plates representing the orbitoBphenoids. The former 

 exhibits on each side, posteriorly and superiorly, a groove, in which 

 the olfactory nerve rests and, above this, expands into an arched 

 process, which supports the anterior extremity of the frontal bone. 

 Anteriorly, the superior end of the bone widens into a rhomboidal 

 plate, which appears externally between the nasal bones. These 

 anterior and posterior processes of the superior edge of the bone 

 are connected by a delicate ridge, which passes from one to the 

 other above, but leaves an irregular oval gap below. 



The anterior edge of the bony plate in question is continued into 

 the unossified septum narium, which below supports the delicate 

 bony representative of the vomer. 



In the chick, the whole of the parts just described are unossified, 

 but the composition and structure of the rest of the axis is essen- 

 tially the same as in the ostrich. 



It is not difficult to identify in the craniofacial axis of the bird, 

 parts corresponding with those which have been shown to exist in 

 the mammal. In the chick, the basioccipital can be readily sepa- 

 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 prespheuoidal cartilage and partially repre- 

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



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

 Ann. ^ Mag, N, Hist, Fb/. 3. Ser. iii. 27 



