6 ROYAL SOCIETY OF CANADA 



than the upper surface, where numerous vascular impressions are con- 

 spicuoua as indicated in the accompanying figures. The markings are 

 accentuated and most numerous on and in the vicinity of the hooked 

 processes and epoccipitals both above and below, and on the upper 

 surface they extend along the axial part forward. A shallow groove, g, 

 figure 2, more clearly shown on the right side of the specimen, extends 

 on the anterior side of the posterior bar from the upper surface near the 

 median line downward and then upward in a regular curve, ending at 

 a point in advance of the base of the hooked process. Above this groove 

 the face of the bar presents a broken surface. On the left side the cor- 

 responding groove is only faintly indicated, and the bone above it is 

 intact. 



Judging from the position and extent of its suture the squamosal 

 was evidently a comparatively small bone. 



The horn core found with the posterior crest of Centrosaurus 

 apertus is apparently from the nasal region; it is straight and laterally 

 compressed so as to be lenticular in transverse section, with a sharp 

 angular edge to the front and rear, recalling the shape of the nasal horn 

 core described by Cope in 1889 under the name Monoclonius spheno- 

 cerus. The specimen is thirty centimetres in length, with the lip 

 missing, and the lower surface imperfect. The compression may be 

 accentuated by distortion, although the fossil does not seem to have 

 suffered at all in that way, nor is there any evidence of injury from 

 pressure in the posterior crest. The surface is marked by numerous 

 vascular impressions and one side is deeply fluted longitudinally as in 

 figure 3. 



Measurements. 



M. 



Extreme length from anterior end of crest (imperfect), medially, 



to line touching posterior edges of specimen on either side "616 

 Length on median line, from anterior end to posterior border. . -486 



Semi-breadth of specimen on curve of under surface '470 



Semi-breadth of specimen horizontally '439 



Vertical drop of lateral edge of specimen IjoIow median line of 



upper surface at mid-length '186 



Antero-posterior diameter of fontanelle "296 



Transverse diameter of fontanelle '248 



Circumference of base of left posterior spur '172 



The conclusion to be arrived at is that Centrosaurus apertus had a 

 broadly expanded squamoso-parietal crest composed mainly of the 



' Horned Dinosaurs of the Laramie. The American Naturalist, vol. xxiii., 

 p. 715, pi. xxxiii., fig-s. 2, 2a. 



