OSTEOLOGY OF THE ARMORED DINOSAURIA. 



69 



Ulna. — On the proximal end the ulna is expanded and massive, but from above 

 downward it tapers gradually to the very much smaller and little enlarged distal 

 end. There is a heavy olecranon process which is produced far above the articular 

 surface for the humerus (o., fig. 35). The surface of this process is roughened for 

 muscular insertion. In S. sulcatus No. 4937 (pi. 20, fig. 3 A), an exceptionally large 

 and aged individual, this rugose area is of triangular shape and extends 

 far down on the posterior side of the shaft. In young individuals, 

 as shown by several specimens, the olecranon is truncate and rises 

 only a short distance above the articular surface of this end. (See 

 pi. 20, fig. 3 B.) In the great development of the olecranon process 

 the ulna of Stegosaurus resembles that of Triceratops, where, among 

 dinosaurs, it reaches its maximum development. 



Viewed proximally the end is subtriangular in outline with a deep 

 transversely concave cavity on the front for the reception of the 

 rounded head of the radius. When these elements are articulated, 

 the ulna completely hides the upper end of the radius from a posterior 

 view. On the inner or radial surface near the distal end is a flattened 

 rugose area, which in an articulated limb is opposed by a similar sur- 

 face on the outer side of the radius. These doubtless indicate the 

 points of attachment for the ligaments that held these bones in place. 



The distal end is comparatively smooth and rounded. In outline 

 it is elliptical, with an area less than that of the radius. 



In specimen No. 4934 the ulna exceeds the humerus in length. 

 In adult animals it was found in nearly all instances that the ulna was equal to, 

 if it did not exceed, the humerus in this measurement. In immature specimens, 

 however, on account of the shortness of the olecranon process the ulna is always 

 the shorter of the two bones. (PI. 20, compare A and B, fig. 3.) 



Measurements of ulnae. 



Fig. 35.— Left 

 ulna of Stego- 

 saurus ungu- 

 latus Marsh. 



tV NAT. SIZE. 



Side view, o, 

 ole ceano n 

 process. After 

 Marsh. 



Greatest length 



Greatest width proximal end 



Greatest width distal end 



Least width of shaft 



Height of olecranon above humeral articulation . 



S. stenops, 

 type No. 



mm. 

 540 



5. sulcatus, 



type No. 



4937. 



mm. 

 594 

 229 

 118 

 61 

 127 



Radius. — The radius, although more slender than the ulna, is nevertheless of 

 robust proportions. The shaft is constricted medially, while the ends are expanded, 

 more especially the distal, which exceeds that of the ulna. In uncrushed specimens 

 the shaft of the bone is a little curved, with the convexity forward and inward. The 

 proximal end is subelliptical in outline, with a slightly roughened concave surface. 

 51873°— Bull. 89—14 6 



