34 



THE CERATOPSIA 



Vascular impressions are present but not very numerous on the upper surface of the squamosal 

 in this adult specimen. This is true also of the type of C. flexus. 



Parietal 



About two-thirds of the area of the crest is formed by the coalesced parietals. Each incloses a 

 large fenestra with a thin, somewhat variable margin. These have much the same extent in all 

 Centrosaurus skulls. The margin is, however, practically never complete in any Centrosaurus skull 

 as preserved, for its ultimate edges must have been extremely thin. The right fenestra of the Yale 

 specimen is materially smaller than the left, although the width of the bone on the outer side is the 

 same in each instance. The left fenestra is somewhat ovoid, with the apex pointing to the rear, and 

 there is an angular prominence in the middle of the anterior margin, but slightly indicated on the 

 right side. The bony edges may in life have merged into a cartilaginous or membranous sheet which 

 covered the aperture, the evolutionary tendency in this Monodonius-T riceratofs phylum being 

 toward the final closure of the fenestra by bone, as opposed to its persistence in the Chasmosaurus- 

 T orosaurus group. 



The supratemporal fossae are large, and open directly backward, due to the overgrowth of the 

 false roofing of the skull. They are bounded by the postfrontals in front and above, and the parietals 

 within and behind. In the rear of each there is a smooth area, depressed below the level of the post- 

 frontal and squamosal bones, which must have formed the place of origin of the temporal muscles. 

 The latter ran obliquely forward and downward beneath the facial bones to the coronoid process 

 of the lower jaw. This is true of all Ceratopsia (see Lull 5 on the musculature of the ceratopsian 

 skull). Bloodvessels also doubtless emerged from the supratemporal fossae to deploy on the dorsal 

 surface of the crest, especially on the anterior and outer portions, but not toward the middle of the 

 crest. 



The central portion of the parietals between the muscular depressions and the fenestrae forms 

 a thickened bar, convex above and to a less extent concave below, which supports the median portion 

 of the crest. Posteriorly, this bar increases in thickness and widens out into the marginal portion of 

 the crest, thus forming the inner and posterior margins of the fenestrae. It terminates between the 

 hook-like posterior processes. 



The free outer margin of the parietals bore four epoccipital bones on either side, the curious 

 inwardly curved hook-like processes constituting a fifth pair, if indeed they are homologous with the 

 epoccipitals. 



In several Centrosaurus skulls, notably in the one in Ottawa (No. 348 G.S.C., which I would 

 also refer to C. flexus), there is a triangular bone filling a corresponding gap at the marginal end of 

 the squamosal-parietal suture. This also seems to be in the nature of an epoccipital, or possibly two 

 fused together, and corresponds to the epoccipital which overlies the end of this suture in Triceratops. 

 In the Yale skull this element is not in evidence, but the space where it may have been is present in 

 the type of the species (No. 5239 A.M.N.H.). The latter has the same number of parietal 

 epoccipitals as the Yale specimen. 



Crest 



The crest or frill differs from that of Triceratofs in several important details. Its general form 

 is more highly arched transversely when viewed from the rear ; the squamosals form a smaller por- 

 tion of the entire area; there are large fenestrae through the central parietal bone; and normally 

 there are two pairs of curious processes arising from the rear of the crest. Of these, the posterior 

 two are small and hook-like, lie in the plane of the crest, and point toward each other, curving for- 

 ward toward their tip. These are seemingly constant in occurrence but variable in form. The area 

 of the crest which lies between them is smooth and rounded. The larger anterior processes are 



5 Lull, R. S., 1908, pp. 388-389. 



