320 SCOTT. [Vol. V. 



figure, it has not the peculiar flaring external condyle which is 

 so characteristic of Mesohippus. 



In the horse, the shaft has become much stouter and 

 straighter, as well as longer than in the Miocene genus, and the 

 ridges for muscular attachment, especially the deltoid hook, 

 much better developed. The external tuberosity is little, if 

 any, larger than the internal, and does not project over the 

 bicipital groove, which, in its turn, has become very broad, and 

 is divided into two portions by the large bicipital tubercle. The 

 latter is now as prominent as either of the tuberosities. The 

 distal trochlea is broad, set nearly at right angles with the long 

 axis of the shaft, and of uniform height. The inner condyle is 

 proportionately more extended than in Mesohippus, and the 

 intercondylar ridge more thickened. The external condyle has 

 not the peculiar facet above described, and there is no supra- 

 trochlear foramen. The inner margin of the anconeal fossa pro- 

 jects far behind the outer. 



The humerus of Mesohippus is in many ways like that of 

 Pcebrotherium, while that of Equus is very similar to that of the 

 camel, especially in the construction of the proximal end. 



The ulna (PI. XXIII., Fig. 23) of Mesohippus, though much 

 reduced, is usually distinct from the radius throughout ; in some 

 specimens, however, the distal ends of the two bones are co-ossi- 

 fied for a short distance. The olecranon is quite high, but thin 

 and compressed, with a thick end and abruptly truncated superior 

 margin ; it is continued upward in a vertical line with the shaft, 

 hardly projecting at all backward. The sigmoid notch is shal- 

 low, and the articular facets for the humerus are confined to its 

 superior and anterior faces, and are obliquely placed with 

 reference to the long axis of the shaft. The radial facets are 

 two, of which the outer one is decidedly the larger and more 

 prominent. The shaft is very slender and compressed, but not 

 interrupted, and the distal end is but slightly expanded and 

 bears a small saddle-shaped facet for the cuneiform. 



The radius (PI. XXIII., Figs. 23-25) is short and has a shaft 

 which is arched forward and quite slender, though in the latter 

 respect there is considerable variation, some specimens being 

 decidedly stouter than others. The shaft is much flattened an- 

 tero-posteriorly, and is of transversely oval section and nearly 

 uniform diameter, except at the extremities. The proximal end 



