FOSSIL HORSE REMAINS FROM IDAHO — GAZIN 307 



•with the transverse processes of the sixth lumbar is about equal to 

 that in the E. grevyi skeleton, distinctly less than in the E. caballus 

 material. 



The caudals show little of interest; however, in comparison with 

 recent material the neural arch is apparently completed in a greater 

 number of the proximal elements. In several individuals the first 

 one or two caudal vertebrae are fused with the sacrum (pi. 31, 

 fig. 2). 



Sternum and ribs. — The segments of the sternum are preserved in 

 only a few individuals, and these are badly crushed and distorted 

 but apparently are not otherwise distinguishable from modern 

 material. A certain quantity of fragmentary material representing 

 sternal ribs is included in the collection, but these elements, being 

 essentially cartilaginous in the animal during life, are very spongy 

 and without significant shape. 



The size of the ribs in these animals appears to have varied con- 

 siderablj^ between individuals. Some correspond favorably in length 

 and curvature with those in the E. grevyi skeleton; in other speci- 

 mens the ribs are larger than in the Arabian horse. Generally, how- 

 ever, they appear relatively broader than in either form and perhaps 

 more deeply grooved on the inner side and externally, although 

 these cavities may have been emphasized largely by crushing. The 

 first few ribs in most cases are shorter and heavier than in the Arabian 

 skeleton. 



Scapula. — The scapulae vary appreciably in size but perhaps aver- 

 age shorter than in E. caballus. Few differences were observed in 

 the fossil material to distinguish it from modern forms. The glenoid 

 surface for articulation with the humerus appears relatively long 

 antercposteriorly, in part because of the forward extension of the 

 anteromedian portion of this surface. In E. grevyi and E. caballus 

 the glenoid surface appears transversely wider, Avith the inner por- 

 tion short anteroposteriorly and the portion external to the glenoid 

 notch projecting more forward of that inward from the notch. More- 

 over, in the fossils a heavier ridge extends from the inner half of the 

 glenoid surface to the coracoid process, approaching in strength the 

 ridge between the outer portion of the glenoid surface and the large 

 tuber scapulae, giving this region a wider anteroventral aspect. 

 A depression of varying markedness extends from the glenoid notch 

 to the coracoid dividing the tuberosity into two processes. An ap- 

 proach is seen in the development of the inner ridge between the 

 glenoid surface and coracoid process in E. grevyi. It is least devel- 

 oped in the E. caballus scapulae observed. 



Humerus. — The humerus (pi. 32, figs, 3, 4; pi. 33, figs. 3, 4) cor- 

 responds closely in size to that in E. grevyi and is more uniform than 

 in E. caballus, which commonly possesses a much larger humerus. 



