184 • Annals of the Carnegie Museum. 



The Radius and Ulna. — The radius and ulna are reduced in length, 

 but otherwise not modified so as to differ in any important particular 

 from those of the earlier species of this family. The most noteworthy 

 difference is the shortness, and perhaps the flatness of the shaft of the 

 radius, which in this species has an oval cross-section half-way between 

 the proximal and distal ends, much as in the later ruminants. The 

 head of the radius has the usual great transverse expanse, and is in 

 all important particulars characteristic of the family. The shaft is 

 rather straight, and, as stated above, compressed antero-posteriorly 

 and expanded transversely; the radial face is straight, while the ulnar 

 and posterior faces are slightly concave vertically. On the ulnar side 

 of the shaft, above the articulation for the ulna, is a sharp ridge, which 

 extends half-way to the proximal end, where it fades away gradually 

 so that the border becomes more rounded; a character quite similar to 

 that in Siis, while the tubercle for the biceps tendon above is much more 

 prominent and more like that in the ruminants. Distally the radius 

 is expanded fully as much transversely, and much more antero- 

 posteriorly, than the proximal end. The scaphoid facet is perhaps 

 even more oblique than in Promerycochceriis montanus, but, as in the 

 latter, it is strongly concave transversely and rapidly flexed back high 

 up upon the radial angle of the bone. The lunar facet is subtriangular 

 in outline with the apex directed backward, and its surface is saddle- 

 shaped, i.e., convex from side to side and concave from before back- 

 wards. 



The ulna is, as usual, not reduced and very heavy, especially above. 

 The olecranon is strongly produced above the sigmoid cavity, but the 

 free end, which is very heavy, is not grooved by a tendinal sulcus as 

 in some of the smaller species of Merycoidodon.^"^ Agriocochceriis 

 latijrons is another Oligocene form which has this tendinal groove;^' 

 and in later forms {Mesoreodon chelonyx^^ and Promcrycocha:nis mon- 

 tanus) the groove is quite distinctly formed. 



The humeral articulation of the sigmoid cavity is broad above and 

 rapidly contracts below, the lip of the inferior articular surface ex- 



'- In the articulated skeleton of Merycoidodon culbertsoni in the Carnegie Museum 

 the tendinal sulcus is not present, while smaller species of that genus have the 

 groove present. 



13 Wortman, Bull. Amer. Miis. Nat. Hist., Vol. VII, 1895, p. 156. 



" Although Scott does not speak of this groove in the olecranon of the ulna of 

 this species, it is well shown in the illustration, PI. X, Fig. 38, Trans. Amer. Pliilos. 

 Soc, Vol. XVII. 1893. 



