82 The Ottawa Naturalist. [J^'ly 



The fontanelles of the crest of Centrosaiirus apertus are in- 

 cluded entirely within the parietal part of the expansion, those of 

 Monoclonins canadensis are bounded on the outer side by both the 

 parietal and the squamosal. 



The crest of C. apertus is composed principally of the coalesced 

 parietals which form an expansion having- somewhat the shape of 

 a saddle, broader than long and much more robust posteriorly 

 than in front where the bone is decidedly thin. The squamosal is 

 not known. The parietal part ot the expansion, figures i and 2, 

 plate I, is longitudinally ridged in the median line, is broadly ex- 

 panded laterally on either side, and ends posteriorly in a robust 

 transverse bar that is concave in outline behind as viewed from 

 above. The fontanelles occur one on either side of the median 

 line and are ot large size: they are bounded behind by the trans- 

 verse bar that forms the posterior border of the crest, and laterally 

 and in front by the thin side extensions. Along the median line 

 the bone is transversely concave beneath. The separate ossifi- 

 cations, named by Marsh epoccipitals, are well developed in four 

 pairs, with in addition the pair of hooked-processes, already 

 mentioned, which are regarded as specially developed epoccipitals. 

 The alar extensions are referred to in the original description as 

 the squamosal portion of the crest, th2 squamosals being then re- 

 garded as having coalesced with the parietals. Near the anterior 

 border of the right extension, however, there is a definite line of 

 demarcation, «, figures i and 2, which can be considered only as 

 the suture for the squamosal. The writer's attention was drawn 

 to this suture, and to the wrong- interpretation in the original de- 

 scription of the nature of t'^e side extensions, by Mr. J. B. Hatcher, 

 Curator of the Department of Vertebrate Palaeontology of the 

 Carnegie Museum, Pittsburgh, who, with his intimate knowledge 

 of the Ceratopsidce, is justly regarded as one of the foremost au- 

 thorities on this interesting family. The postfrontal suture, 5, 

 figures I and 2, extends from the inner side of the anterior end of 



the fontanelle obliquely forward and inward to the median line in 

 front. Numerous impressions of blood-vessels are present on and 

 in the neighbourhood of the epoccipitals and hooked-processes, 

 and on the upper surface along the median ridge. The thickness 

 of the bone at a number of points, is indicated in centimetres by 

 the numerals in figure i. 



