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THE AMERICAN NATURALIST [Vol. XL 



All of the bones described above are seen in a dorsal view of the 

 skull; those of the bony cranium that will now be described, are 

 best seen from the ventral aspect. 



The parasphenoid (Pa.) is the only unpaired bone in the skull, 

 and is larger than any other single bone. It forms practically the 

 entire floor of the cranial cavity, as seen from the exterior. The 

 bone as a whole has somewhat the shape of a broad, blunt-pointed 

 dagger, with the point towards the anterior end of the skull, and 

 partially concealed, in a ventral view, by the posterior ends of the 

 vomers, with which it articulates (Pa. in Figs. 2 and 3). Later- 

 ally the parasphenoid articulates, for the anterior half of its length, 

 with the pterygoids. Just posterior to the pterygoids it articulates 

 dorsally with the small prootics, and posterior to the auditory fora- 

 men it articulates with the exoccipitals. Its extreme posterior 

 end forms the ventral border of the occipital foramen. A short 

 distance anterior to this point is seen, in some specimens, a well 

 defined, irregular transverse line (Fig. 2), which would seem to 

 indicate the presence of a basioccipital bone, but as no separation 

 of the bone along this line could be effected, the presence of a de- 

 finite basioccipital could not be determined. On each side of the 

 paras])henoi(l, close to its point of union with the prootic, is a canal 

 (('. ('. ), leading into the cranial cavity, called by Osawa the caro- 

 ti.l canal. XeanT the midventral line, somewhat anterior to the 

 jirecedin;^, are two very small openings (C), probably for the en- 

 trance of inimite blood vessels. The ventral surface of the para- 

 sphenoid is smooth and nearly flat, except at the anterior end where 

 it is more or less convex, with a slight median ridge that fits in 

 between the posterior ends of the vomers. 



The romcrs (f'.i are the large flat bones that form the base of 

 the anterior end <.f the skull and the floor of the nasal cavity. 



