[parks] CENTROSARUS APERTUS 55 



The two skulls at Ottawa differ considerably in detail but the 

 relation of the bones seems to be identical. The chief difference is 

 that the forwardly directed processes over the fontanelles are in one 

 case fairly straight, as in the type, but in the other are much more 

 hook-like, as in the present example. Both these skulls are larger 

 and more robust in all their parts than the specimen under review. 

 Assuming the identity of Monoclonins flexiis and Centrosaurus apertus 

 a comparison will be made with the measurements given by Brown 

 for the former and with those of the skull figured by Lambe for the 

 latter. In ascribing the present specimen to either of these species 

 it is at once admitted that certain differences exist. The nasal horn- 

 core is not so distinctly curved forward, the frill is relatively narrower, 

 the fontanelles smaller, and the forwardly directed processes over 

 the fontanelles much less robust. The uppermost sinuosity of the 

 frill margin is not drawn out to a prominent point. The epoccipital 

 ossifications are more distinct and a separate marginal bone between 

 the parietal (?) and squamosal is only faintly indicated. The orbit 

 is less circular and its greatest diameter differently disposed. The 

 suture between the jugal and the squamosal is less vertical, and the 

 lateral temporal fossa has its longer diameter much nearer vertical 

 in position. The lachrymal is higher than that shown in Lambe's 

 figure and there is no distinct indication of the suture with the post- 

 frontal. None of these differences seem to be of sufffcient importance 

 to justify a new species and may easily be accounted for by the less 

 advanced age of our skeleton. In fact, there is sufffcient difference 

 in the two sides of the frill to equal in importance the differences 

 observed above. The parietal part of the frill on the left side shows 

 a peculiar overlap in the bone outside the fontanelle and this orifice 

 is itself much smaller than on the other side. Whether this is a mere 

 variation in growth or whether it is due to an injury during the life 

 of the animal it is difficult to say. 



Measurements indicate that the cranium of our specimen is 

 slightly more than four-fifths the size of that of Monoclonins flexiis, 

 as the following table will indicate. 



