54 



THE CERATOPSIA 



Hatcher's description of that of Monoclonius crassus™ The humerus is a heavy bone with a very 

 prominent deltoid ridge which is continued into the radial crest and extends more than halfway 

 toward the distal end. In Triceratofs the radial crest bears a separate rugosity quite distinct appar- 

 ently from the deltoid ridge, whereas in Monoclonius including Centrosaurus the two combine more 

 or less into the deltoid ridge. The head is a comparatively small, almost hemispherical eminence 

 which lies within the median half of the proximal end and looks backward at right angles to the shaft 

 of the bone, so that the humerus during life must have been nearly horizontal in position. In this 

 respect it agrees with that of Chasmosaurus, but differs from that of Triceratofs in which the head 

 seems to merge into the summit of the bone, implying that the normal angulation of the shoulder 

 was greater than a right angle. 



Another crest comparable to the deltoid ridge lies on the extensor side of the head or the 

 humerus but continues little more than half as far as does the former. Distally, the trochlear end 

 of the humerus is well developed, the radial condyle being the more prominent. Here again there is 

 indication that extreme flexion of the elbow joint was habitual. 



There is little if any distinction between the humerus under discussion and that of M. crassus 

 except that the latter seems more robust. It differs markedly from Triceratofs in its greater slender- 

 ness and especially in the conformation of the proximal end. 



Dimensions of Humerus 



C . flexus 



No. 2015 Y.P.M. 



left 



Length 585 mm - 



Width across radial crest 180 



Least width of shaft 



Width across trochlear end 195 



Length of deltoid ridge and radial crest 275 



A B 



Y.P.M. 2015 



Fig. 22.— Left ulna of Monoclonius {Centrosaurus) flexus. A, posterior view; B, anterior view; 1/6 natural size. 

 f, proximal extremity; d, distal extremity; o, olecranon; r, surface for proximal end of radius. 



26 Hatcher, Marsh, Lull, 1907, pp. 79-80. 



27 Estimated from figure. 



