No. 3.] OSTEOL OGY OF MESOHIPPUS AND LEPTOMER YX. 327 



VI. The Hind Limb. 



The pelvis (PI. XXIII., Fig. 31) of Mesohippus is very like 

 that of the horse, with some differences of detail. The neck of 

 the ilium is much longer and more slender, when viewed from 

 the side, but rather broader when viewed from below, owing to 

 the prominence of the pubic border, upon which a small rugose 

 spine is developed. The plate is too much injured in the only- 

 specimen at my disposal to allow an accurate determination of 

 its shape, but it is plain that it expands much less abruptly than 

 in Eqitus, and apparently it is relatively much smaller than in 

 that genus. The acetabulum is deep, with prominent edges, 

 and the pit for the round ligament is smaller and more median 

 in position than in the modern forms. 



The ischium is long and quite straight ; for most of its length 

 it is much more slender and rod-like, and less curved upward than 

 in Equus. The posterior portion is expanded and depressed, 

 sending off from its outer side a long and sharp but low tuber- 

 osity. Above the acetabulum the border of the ischium is very 

 slightly elevated into a crest, which terminates behind in a shal- 

 low groove for the internal obturator tendon. The obturator 

 foramen is very much larger propor- 

 tionately than in the living forms, and 

 consequently the posterior expansion 

 of the ischium much smaller. 



Kowalevsky states that in Anchi- 

 therium the crest of the ischium is 

 more rounded and thickened than in 

 the horses, and that the sulcus for 

 the internal obturator tendon, which 

 is wanting in the horses, is profound 



* , r . Fig. A. — Right femur of Meso- 



The femur of Mesohippus (Fig. A) hippus . anterior view _ xf- 



is characteristically equine in appear- 

 ance. The shaft is quite long and stout, in comparison with 

 the other long bones, but slender as compared with the mod- 

 ern type. When viewed from the side, it presents a decided 

 sigmoid curve. The head is small and nearly sessile, but rises 

 somewhat above the bridge connecting it with the great tro- 

 chanter : the pit for the round ligament is deep, but narrow and 



