THE FORE LIMB AND FOOT 55 



Ulna 



(Fig. 22) 



The general character of the ulna is not unlike that of Triceratops except that it is much less 

 robust, and the olecranon process, as preserved, is much less pronounced, occupying little more than 

 one-third the total length of the bone, whereas in Triceratops, measured in the same way, it is about 

 one-half. The ulna of Centrosaurus has a somewhat curved shaft, flattened distally. As preserved, 

 there is a longitudinal hollow on the inner face and a corresponding low ridge on the outer face which 

 disappears in the distal third of the bone. Proximally, this ridge culminates in a high crest which, 

 together with the anterior margin of the bone, bounds a deep depression to receive the proximal end 

 of the radius. The olecranon seems to be defective as it does not begin to show the massiveness of 

 that of Triceratops, either relatively or actually. 



Dimensions of Ulna 



, — C. flexus Y.P.M. — , 

 left right C. nasicornus 



Length over all 437 mm. 457 mm. 450 mm. 



Greatest width 



Proximal end '56 



Distal end 96-5 



Radius 



(Fig. 23) 



Both radii are fairly well preserved, especially the left. The radius is a slender, slightly curved 

 bone dilating proximally to form the humeral facet, the superior aspect of which is concave to receive 

 the radial condyle of the humerus. Distally, the radius is somewhat flattened. At mid-shaft, on 

 the other hand, it is trihedral in section. Several roughened areas occur for muscle attachment. 



Y.PH 2013 



Fig. 23.— Left radius of Monoclonius (Centrosaurus) flexus. A, posterior view; B, anterior view, 1/6 natural size. 



f, proximal extremity; d, distal extremity. 



