308 PROCEEDINGS OF THE NATIONAL MUSEUM vol.83 



The fossil humeri are distinctly Equus-like but show a few differences 

 from the modern form. The ridge between the bicipital grooves 

 on the anterior surface of the proximal end is distinctly less outstand- 

 ing (pi. 33, figs. 3, 4) and somewhat shorter longitudinally than in 

 Equus. The greater or lateral tuberosity projects anteriorly to a 

 noticeable extent although relatively not so much as in earlier horses. 

 The ridge external to the bicipital grooves continues distally a short 

 distance on the anterior surface somewhat more prominent on the 

 average than in Equus grevyi, and the anterior face of tliis portion 

 of the humerus may be somewhat narrower. The deltoid tuberosit}^ 

 is prominent but generally not so rugged, and the rugose portion 

 is not developed longitudinally to the extent seen in mature E. 

 grevyi humeri. On the distal portion of the humerus the epicondyles 

 appear in most specimens to close proximally over the olecranon 

 or supratrochlear fossa somewhat more acutely than in the few humeri 

 of E. grevyi observed. 



Radius and ulna. — The fused radius and ulna (pi. 32, figs. 7, 8; 

 pi. 33, figs. 7, 8) are about the size of those in E. grevyi, somewhat 

 shorter than in most E. caballus, and show little to distinguish them 

 from the corresponding parts of the modern forms. The olecranon 

 is relatively deeper fore and aft and perhaps somev/hat shorter in 

 length on the average, and the inner surface shows a broad excavation,, 

 which is much more conspicuous than in modern ulnae observed. 

 The shaft of the ulna is much reduced, the degree of reduction as in 

 the Hving horses varying from a continuous sliver to complete absence 

 from most of the distal half of the radius. The distal extremity of 

 the ulna is fused with the radius and articulates with, the cuneiform 

 and pisiform. The width across the articulating surfaces on the 

 distal end of the radius in most of the fossil specimens is less than 

 in the few modern radii available. 



Carpus. — The proximal or radial surface of the scaphoid is strongly 

 flexed with the posterior portion wide and turned upward posteriorly. 

 The notch on the lunar side of the radial surface of the scaphoid is 

 deep and placed about as in specimens of E. grevyi. In E. caballus 

 the notch appears shallow and nearer the posterior margin. On the 

 external side the distal facet for the lunar appears larger in the fossil 

 material than in Equus. On the distal surface the confluent facets 

 for the trapezoid and magnum are generally somewhat constricted 

 anteroposteriorly at the line of contact. The inner or medial surface 

 usually shows a strong tubercle anteroproximally as in E. grevyi but 

 more prominent than in the E. caballus scaphoids observed. 



The posterior concavity on the proximal surface of the lunar is 

 strong as in the scaphoid and shows less tendency to be convex trans- 

 versely than in E. caballus and forms a sharper angle with the portion. 



