86 THE CERATOPSIA 



On the basis of these details, I recognize two clearly defined species, flexus and nasicomus, 

 either of which may be referable to the genoholotype apertus. But in view of the paucity of this 

 type, it merely serves to establish the sub-genus as compared with Monoclonhis and not the species, 

 hence no reference of any other skull to this species can be made with any assurance at all. It is the 

 custom in the Canadian museums to label as apertus all exhibition specimens of Centrosaurus, the 

 correctness of which can neither be affirmed nor denied. I shall try to allocate each to one or the 

 other of Brown's species, with the following results. 



Monoclonius (Centrosaurus) flexus Brown 17 



Pis. II, III; Text Figs. 4, 5, 7-10, 13, 14, 16-29 



Holotype: No. 5239 A.M.N.H.; skull. 



Horizon: Belly River formation. 



Locality: About 1 mile below Steve ville, on the east bank of Red Deer River, Alberta. 



Collector: American Museum expedition of 1912. 



Plesiotype: No. 2015 Y.P.M. 



Horizon: Belly River formation. 



Locality: 12 miles below Steveville, 2 miles from the mouth of Sand Creek, on the east bank. 



Collector: American Museum expedition of 1914. 



The chief distinguishing character of this species lies in the nasal horn which is long and 

 curved forward. 



To this species I would also refer skull No. 348 G.S.C., at Ottawa, although it departs from 

 the definition in the extreme curvature of the nasal horn and anterior processes, and in the decided 

 rugosity of the premaxillary profile of the muzzle. These may be due in part, however, to age. 

 Skull No. 347, at Ottawa, agrees in the forward curvature of the nasal horn, but disagrees in the 

 rudimentary character of the brow horns, and in the straight anterior processes. This last detail 

 corresponds with the apertus type, beyond which comparison fails. 



Monoclonius (Centrosaurus) nasicornus Brown 18 



PI. IX, A; Text Figs. 11, 12, 15 



Holotype: No. 5351 A.M.N. H.; a complete skeleton. 



Horizon: Belly River formation. 



Locality: North fork of Sand Creek, 12 miles below Steveville, Red Deer River, Alberta, Canada. 



Collector: American Museum expedition of 1914. 



The skull characters of this type are as follows: Nasal horn erect, brow horns rudimentary, 

 being represented by a heavy rugosity above the orbits (whether or not the actual horn core was 

 detached in maceration can not be proved), crest processes moderately curved, muzzle deep, profile 

 very convex and rugose. 



To this species, besides the holotype, No. 5351 A.M.N.H., I would refer the skull No. 8897 

 U.S.N. M. in which the nasal horn, so far as preserved, is erect, although restored by the collector, 

 C. H. Sternberg, as curving slightly forward. Otherwise, it conforms well with the type. Another 

 skull, No. 4519 R.O.M., Toronto, agrees with the type in its erect nasal horn and rugose muzzle, 

 although the latter is not so extreme as in the type. The brow horns, on the contrary, do not corre- 

 spond, as they are pointed and trihedral, being about as well developed as in any Centrosaurus. 



17 Brown, B., 1914 B, pp. 549-5 58. 



18 Brown, B., 1917, pp. 281-301. 



