312 PROCEEDINGS OF THE NATIONAL MUSEUM vol.83 



Femur. — The length of the femur corresponds to that in E. grevyi 

 or moderate-sized individuals of E. cahallus, but the extremities appear 

 somewhat slenderer on the average (pi. 32, figs. 1, 2; pi. 33, figs. 1, 2). 

 The width of the proximal end across the head and anterior portion 

 of the greater trochanter is generally less, and the neck below the 

 head may be more constricted anteroposteriorly. Between the lesser 

 trochanter and the third trochanter the anterior face of the shaft is 

 commonly narrower or less flattened anteriorly, although not in every 

 case. On the distal end of the femur the distance between the epi- 

 condyles averages less and the articular surfaces of the condyles are 

 slightly narrower. The distance from the anterior surface of the troch- 

 lea to the posterior surfaces of the condyles is also somewhat less in 

 most specimens than in E. grevyi or E. cahallus. It is possible that 

 most of these differences and many of those in other parts of the 

 skeleton would disappear were a larger series of modern skeletons 

 available ; however, it is hkely that in many cases an average difference 

 would be maintained. 



The patellae do not exhibit any characters of special interest and 

 apparently do not differ in any important respect from those in 

 Equus. 



Tibia and fibula. — The tibia (pi. 32, figs. 5, 6; pi. 33, figs. 5, 6) is of 

 moderate size, and, as observed in the femur, the proximal and distal 

 portions are slightly less in diameter than in a large zebra or an Arabian 

 horse. The articular facets average smaller than in the available 

 modern tibiae of about the same length. The cnemial crest appears 

 prominent and rugged as in modern forms. 



In the few fibulae (see figs. 5 and 6 of pis. 32 and 33) preserved it 

 appears, as observed by Matthew in the Blanco material, that 

 reduction has progressed to a greater extent than in modern Equus. 

 The fibula tapers abruptly and where complete is found to terminate 

 at 7 or 8 cm, or a small fraction of the length of the tibia, whereas in 

 the Equus skeletons observed the fibula extends about halfway down 

 the tibia, or somewhat farther. 



Tarsus. — The astragalus is surprisingly conservative in form al- 

 though it shows a range in size variation. Astragali in E. grevyi and 

 E. cahallus correspond closely in detail to those in the fossil form and 

 show no important differences other than perhaps a somewhat greater 

 backward extent of the proximal portions of the trochlear ridges, and 

 a greater size range in the E. cahallus material. 



The calcaneum is about as large as in E. grevyi or an Arabian horse 

 and shows little to distinguish it from Equus. The sustentaculum is 

 less prominent than in the Arabian horse, and the long axis of the 

 sustentacular facet is more nearly parallel to the vertical axial plane. 

 In the specimen of an Arabian horse the sustentacular facet in both 



