SKULL OF A HADEOSAUEIAN DINOSAUR — GILMORE 



489 



it unites with the orbitosphenoid, but their precise relationships are 

 not clearly shown by this specimen. 



Orbitosphenoid. — A pair of small subrectangular bones (see fig. 34, 

 Osp) are identified as the orbitosphenoids. This identification rests 

 to a considerable extent on the presence of several foramina that 

 perforate these bones and that can be homologized with the nerve 

 openings in the orbitosphenoid region in other Dinosauria crania. 

 Unfortunately, the sutural edges have suft'ered from abrasion and 

 crushing and thus offer Httle positive information as to their correct 



Figure 34.— Right orbitosphenoid and ethmoid (U.S.N.M. no. 

 n893). Alsp, sutural border for alisphenoid; Eth, ethmoid; 

 Osp, orbitosphenoid; I, outlet for the olfactory nerve; //, the 

 second or optic foramen; ///, IV, foramen for third and fourth 

 cranial nerves, respectively. Natural size. 



relationships to the surrounding skull elements. A deeply grooved 

 sutural border, having an extensive outer Up for squamous union, 

 fulfills all requirements for a perfect articulation with the ethmoid 

 and thus is regarded as the anterior edge of this bone. The superior 

 edge is especially thickened for articulation with the overlying frontal. 

 The posterior unites with the ahsphenoid, and the ventral with the 

 basioccipital, which is missing in this specimen. Since the basi- 

 sphenoid and parasphenoid are both missing, it cannot be determined 

 whether the orbitosphenoid was m contact with the parasphenoid. 

 A small triangular sutural area on the lower anterointernal side of 

 the orbitosphenoid appears to indicate a surface for union with its 

 fellow of the opposite side on the median line below the forward part 

 of the cerebral hemispheres. 



The orbitosphenoid is perforated by a number of foramina identified 

 as shown in figure 34. 



The foramen for the optic nerve Ues very close to the orbito- 

 sphenoid-ethmoid sutural border, and in this specimen it seems to 

 be a notch or groove on the anterior ventral border of the orbito- 

 sphenoid with the ethmoid contributmg to its boundary. It is quite 

 possible that in better-preserved specimens an external view would 



