OSTEOLOGY OF THE AEMOBED DINOSAUBIA. 31 



forms the anterior boundary of the foramen ovale in much the same manner as in 

 Camptosawnis} The external surface contributes to the inner wall of the supra- 

 temporal fossa, whUe the internal forms a portion of the brain case containing the 

 cerebral hemispheres. 



OrUtosphenoid (or. sp.). — The presence in the skuU of Stegosaurus of ossified 

 orbitosphenoids is clearly shown by two specimens, Nos. 2274 and 4936 (fig. 10, also 

 fig. 2, pi. 10). As shown by specimen No. 4936, upon which the following descrip- 

 tion is based, there are two curved plate-like bones, united ventrally on the median 

 line to form that part of the brain case containing the olfactory lobes. Dorsally 

 they unite exclusively with the frontals, and posteiiorly with the alisphenoids. 

 Above the point where the orbitosphenoids unite in fi-ont is an opening which 

 undoubtedly gave exit to the olfactory nerves. They appear to have left the brain 

 through this single orifice {ol., fig. 2, pi. 10), at least no bony partition separating 

 them has been observed. On the posterior borders of these bones are foramina 

 for the exit of the second, third, and fourth nerves. 



Parietal (p.) — The dorsal portion of the parietal segment of the skuU is formed 

 by the parietal, an unpaired element in Stegosaurus, as in all known dinosaurian 

 skulls. 



Viewed from above, the parietal is a short, heavy bone with laterally expanded 

 ends. The superior surface is flattened and without median longitudinal crest or 

 ridge. The median lateral surfaces drop abruptly, at right angles to the upper 

 surface, and form the inner boundary of the supratemporal fossae. The least 

 transvei-se width of the paiietal between the fossae in No. 4934 measures 34 mm. 

 Anteriorly it unites with the broad frontals by an evenly (in No. 4934) rounded 

 imbricating suture (pi. 6). In other specimens this suture is more angular, as 

 shown in figure 1, plate 10. The antero-lateral angles meet the postorbital and 

 postfrontal and in S. stenops excludes the frontal from the supratemporal fossa. 

 In S. armatus (fig. 1, pi. 10), the frontal participates in the boundary of this fossa. 

 Latero-posteriorly are vertically expanded processes, which extend backward and 

 outward along the top of the supraoccipital, meeting a branch of the squamosal 

 by a lapping suture about the center of the posterior boundary of the supratemporal 

 fossa. The median posterior border of the parietal thins out to a rounded edge, 

 which in most skulls slightly overhangs the underlying supraoccipital. The median 

 ventral surface is hoUowed out transversely and forms the piincipal part of the 

 roof of the brain case. 



Frontal (/.). — ^The fi'ontals are paired bones, although in old individuals the 

 suture coalesces. Above the orbits they are thick and heavy, with a sutural edge 

 15 mm. long between the anterior process of the postorbital and the posterior 

 process of the prefrontal which unites with the postfrontal. They overHe the 

 parietals posteriorly as is shown by specimen No. 6645. Viewed fi-om above the 

 frontal region is flat. From this view each bone sends out a sharp narrow anterior 

 process which unites on the median Hne between the posteriorly directed fingerlike 

 processes of the nasals. The nasal processes overlie the frontals, being received 

 in the longitudinal grooves shown a,t ns., figure 1 , plate 10. External to these grooves 



1 Compare flg. 5, Proc. U. S. Nat. Mus., vol. 30, 1909, with flg. 10, i. e., by the development of a slender process which 

 meets the basisphenoid, well shown in a skull of S. armatus (Cat. No. 4936, pi. 10.) 



