84 The Ottawa Naturalist. [July 



mity is known. This part of the parietal is keeled below, figure 7, 

 and is triangular in cross section, with a flat upper surface, figure 

 5, that fits into a shallow groove along the inner posterior border 

 of the lower surface of the squamosal as shewn by the dotted lines 

 in figures 4 and 6. Its free outer border behind continues the 

 sinuous curves of the outer border of the squamosal. The parietal 

 portion of the crest of this species may have had some resemblance 

 to the coalesced parietal bone, the only known part of the crest, of 

 Monoclonius belli. 



On the inner side of the squamosal and the forwardly directed 

 posterior extension of the parietal was an opening in the crest of 

 great size much larger probably than the parietal fontanelle of Cen- 

 trosaurus apertus. 



From the foregoing it is seen that the posterior crest of 

 Monoclonius canadensis extended far back, that it was made up ot 

 the parietals (probably coalesced and bearing some resemblance in 

 form to the corresponding portion of the frill of M. belli) and the 

 squamosals, the latter entering largely into its composition, and 

 that fontanel'es of very large size were present. We «ind also that 

 the fontanelles were not included entirely within the parietals, as 

 in Centrosaurus apertus but were bounded laterally in front by the 

 squamosals. 



EXPLANATION OF PLATES. 



Plate I. 



Figure i — Coalesced parietals oi Centrosaunis apertus viewed from 

 above; one-eighth natural size. 



Figure 2 — The same, viewed from the right side and similarly 

 reduced. 



Figure 3 — Lateral aspect of nasal horn core of C apertus ; one- 

 eighth natural size. 



Plate II. 



Figure 4 — Right squamosal of Monoclonius canadensis, exterior 

 view ; one-sixth natural size. 



Figure 5 — Upper surface of front end of right lateral posterior ex- 

 tension of parietal of M. canadensis ; similarly 

 reduced. 



Figure 6 — Interior view of the squamosal shewn in figure 3; simi- 

 larly reduced. The dotted line in this figure and in 

 figure 4 indicates the position of the parietal ex- 

 tension when applied to the squamosal. 



Figure 7 — Lower surface of the bone shewn in figure 5; similarly 

 reduced 



