82 THE MAMMALIA OF THE DEEP P.IVEB BEDS. 



This specimen was found by Prof. W. F. Magie in the lower beds of the Deep 

 River valley. 



Miohippus sp. 



What is probably a third species of this genus is represented by teeth and limb 

 bones from the upper beds (lower Loup Fork), the specific reference of which is 

 uncertain in the absence of the characteristic upper molars. In size, this animal 

 about equals the M. (Anchitherium) brachylopilius Cope from the John Day, and is 

 therefore one of the smaller forms which exceed Mesohippus in stature but relatively 

 little. Fragmentary as they are, these remains are of interest as showing the degree 

 in which some members of a genus may retain their primitive characteristics, while 

 descendants of the same genus have already advanced far on the road of specializa- 

 tion. To make clear the position of these late survivors, it will be most convenient 

 to compare the specimens with the corresponding parts of Mesohippus, than which 

 they are distinctly more modernized, though perhaps less than we should have ex- 

 pected. 



The humerus has a somewhat more rounded and less laterally compressed shaft. 

 The trochlea is decidedly more equine, lacking the peculiar flange which I have else- 

 where described as occurring on the external side of the humeral trochlea in the 

 White River genus. This external surface is continued much farther back upon the 

 distal aspect of the trochlea, and thus there is not that conspicuous difference in ver- 

 tical diameter between the external and internal portions of the trochlea which gives 

 such a peculiar and characteristic appearance to the humerus of the older form. It 

 should be added that the humerus of this species is more modernized than that of the 

 John Day members of the genus, which still display some traces of the arrangement 

 found in Mesohippus. Returning to the species under description, we find that the 

 intertrochlear ridge is reduced, but is still much more prominent than in JEJquus, and 

 the sulcus, which in the latter is placed at the bottom of the intertrochlear groove, 

 is on the summit of the ridge ; in Mesohippus it is wanting. The inner portion of 

 the trochlea has its articular surface reflected farther back on the upper side than in 

 the horse, which indicates a greater freedom of motion at the elbow joint. The 

 internal epicondyle is reduced and, seen from this side, the distal end is a reduced 

 copy of that of JUquus. 



Except for the position of the bicipital tubercle, which is still internal rather 

 than anterior, the head of the radius is much more equine in appearance than that of 

 Mesohippus. ]STot only is the peculiar external flange absent, but the head is wider 

 and expanded much as in the horse. The shaft is likewise more flattened antero- 



