BRACHYCKRATOPS MONTANENSIS 99 



riorly, it shows a short suture for the small supraorbital bone which is missing. The horn is situated 

 on the extreme anterior end of the postorbital and not on the supraorbital at all. Gilmore 44 doubts 

 the propriety of designating these horns as supraorbital, but as generally understood, the name 

 refers to their position above the eyes rather than to the element which bore them. The horn core 

 is low, with a summit obtusely rounded longitudinally but sharply pointed transversely. The 

 external surface is plane, the internal one strongly convex, and the antero-posterior diameter greatly 

 exceeds the transverse. The height of the horn above the orbit is 3 1 mm. 



The crest is represented by the median elements from two individuals (No. 7950 and No. 7951 

 U.S.N. M.). For this median element Gilmore 4 ' accepts the designation dermosupraoccipital or 

 interparietal, after Von Huene. It is united by suture with the anterior portion of the skull at the 

 postfrontal fontanelle, articulated with the postfrontals above and questionably with the parietals 

 (our supraoccipital) beneath. The median part is sharply ridged except at the posterior extremity, 

 where it flattens into a thinner portion with an emarginate median border. Between the fenestrae the 

 median bar has a thickened triangular cross-section. The bone surrounding the frill fenestrae is 

 very thin, but it thickens toward the lateral free edges and posteriorly. The exact shape and extent 

 of the frill fenestrae cannot be determined from the available specimens; in fact, judging from 

 Gilmore's figure, there is only a small portion of the antero-internal margin of the right one which 

 can be determined at all. He is probably right in saying that they were comparatively small. In 

 this, as in the short squamosal, Brachyceratofs agrees more with Monoclonius than with Chasmo- 

 saurus. There were no epoccipital bones on the margins of the frill, but a series of prominences on 

 either side of the median emargination give the periphery a scalloped effect comparable to that pro- 

 duced by the epoccipitals of Triceratops. Epoccipitals may have been present, however, and may 

 yet be demonstrated by the discovery of a more mature skull. 



As Gilmore 4 '' rightly says, "The modifications found in the ceratopsian skull are greater perhaps 

 than in any other known group of reptiles, living or extinct. . . . The inclosed and compact nature of 

 the skull was of the greatest value as a means of protection, and it contrasts strongly with the open 

 structure of most other dinosaurian skulls. These modifications have led to a rearrangement of the 

 elements of the cranium, that until interpreted is as confusing as it is unusual. . . . 



"It now appears that the prefrontals and postfrontals, which in the normal reptilian cranium 

 are lateral to the parietals and frontals, have in the ceratopsian skull pushed upward and inward 

 above those bones, completely covering their dorsal aspect," 4 ' although they are not in contact with 

 them because of the intervening frontal sinus. 



Dimensions of Skull 

 (After Gilmore) 



Greatest length, about 565 mm. 



Greatest breadth, estimated 400 



Length of crest along median line 315 



Height of nasal horn core above nares 125 



Breadth between center of orbits 130 



Postorbital horn cores at base 



Anteroposterior diameter 34 



Transverse diameter 27 



Horizontal diameter of orbit 57 



Nasal horn core at base 



Anteroposterior diameter 90 



Transverse diameter 41 



"Gilmore, C. W., 1917, p. 11. 



45 Loo cit., p. 1 1. 



48 Loo cit., pp. 17-18. 



47 Our usage except* the parietal which forms the median element of the crest. 



