546 



ON THE THEORY OF THE VERTEBRATE SKULL 



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 

 posterior 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 orbitosphenoids. 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. 



Fig. A. — Longitudinal section of tiie Skull o^ a young Ostrich. 



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 essentially 

 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 separated 



