-Ul K L. Troxdl — Early Pliocene One-Toed Horse. 



Metacarpal IV has a diameter greater by l-5 inni than that of 

 II. This difference in size is the character upon which Marsh 

 Bought to found Pliohippus gracilis, hut the species has been 

 ruled out (Lull, Gidley) and the feature upon which it had 

 been established is considered of little importance except as it 

 signifies the unsymmetrical reduction of the foot. The dis- 

 symmetry of the foot is also shown by the presence of a small 

 nodule of bone representing the fifth metacarpal ; while on 

 the other side the remnant of metacarpal I is not even posi- 

 tively known in the earliest forms of Eohippus, showing that 

 the first digit was much more progressive in its reduction. 

 The vestigial metacarpal V has two articular facets joining it 

 with the unciform and metacarpal IV. It harps back to the 

 period of Eohippus, when for the last time the front foot bore 

 the phalanges of a fourth toe. 



Third phalanx. — The hoof of Pliohippus lullianus is very 

 flat, especially in comparison with that of Equus scotti or E 

 caballus. The angle of the front slope in the new type is 

 about 27^° and in this respect approaches that of Mesohippus : 

 about 22-^-°. In Equus this angle measures over 45°. The 

 feet have a different structure as a result: in P. lullianus and 

 the earlier genera the second phalanx hangs on the back of the 

 hoof with no part of the articular surface horizontal, while in 

 the modern genus a part of the weight transmitted falls 

 directly upon the hoof bone, a device better suited to the 

 greater weight. The cleft in the anterior border of the ungue 

 of the new type is a very primitive feature, one not found in 

 Equus but common in the earlier forms. 



Measurements in millimeters : — 



Cannon bone Asinus P. lullianus E. scotti 



ratio % ratio % 



Width distal 35-8 84 30 52 bl'1 



Diameter of keel, ant-post 27-8 94 26 61 42-7 



Shaft transverse 22-0 86 19 46 41-0 



Proximal width 38-5 88 34 59 57-3 



Proximal ant-post 25-3 99 25 60 41-5 



Length 191-0 115 219 90 244-0 



Speed index 8*68 11-53 5-95 



First phalanx P. lullianus Second phalanx P. lullianus 



Proximal width.... 32 Proximal width. 31 



Distal width 29 Proximal, ant-post 20 



Shaft width 19 Length 26 



Proximal, ant-post. 25 



Shaft, ant-post 12 Third phalanx, hoof 



Length 56 Altitude 20 



Width 40 



Length 36 



