248 FROM OROHIPPUS TO MIOHIPPUS chap. 



has a well-marked coracoid process. The radius and ulna are 

 separate ; so too are the tibia and fibula. Eohippiis, belonging 

 to the same sub-family, is slightly more primitive ; for the hind- 

 feet have a rud.iment of digit I. Oroh-ippms is a little nearer to 

 the Horses in that the molar teeth have acquired a little further 

 advance towards the equine type. Instead of the tubercles of the 

 teeth remaining for the most part separate, they have fused into a 

 set of ridges, of which, however, the pattern is less complex than 

 in the modern Horses. In other respects Oroltipims is much hke 

 Hijrucotlierium. Pachynolophus seems to be but a synonym. 



The next stage is shown by Mesuhippus, a Lower ]\Iiocene 

 form, usually referred to the neighbourhood of Palaeoth erium. It 

 has nearly lost one of the toes of the fore-foot, a rudiment only 

 remaining ; the metapodials, at any rate of the fore-feet, seem to 

 be slightly increased in length. The orbit is not encircled by 

 bone, but there is a strong process from the frontal, which nearly 

 meets the zygomatic arch. 



Anchitherium, from the Upper iliocent', is not far removed in 

 structure from the last-mentioned form ; it is a trifle nearer the 

 existing Horse in several points. Tlie ulna is further reduced and 

 fused with the radius below ; the rudiment of digit V is still 

 more rudimentary ; the two lateral digits are smaller in proportion 

 to the central one than they are in Mesohip2yus ; the fibula is 

 fused below with the tibia. From this form to Equus is a small 

 series of steps, characterised by the still further reduction of all the 

 digits except III, by the still further reduction of the already 

 rudimentary ulna and fibula, and by the increasing depth of the 

 molar teeth, which are of com-se, in Equus, hypselodont. 



Another interesting conclusion may seem to follow when we con- 

 sider the geographical range of the ancestral Horses. Hyrai-otlicrknn 

 and Pachynolophus occm'red both in the Old and New World. 

 From them may have arisen the Horses of both hemispheres. 

 After that point there is a division. }[rsohipp)us is American, 

 and we get at Equus in that continent through Desmatippus and 

 Protohip>pus. On the other hand there are no remains known of 

 Mesohip)pus in Europe ; and unless subsequent researches prove 

 the existence of Mesohippius, we have to rely upon forms which 

 are placed with Anchitherium and Hip)parion. 



It seems that in America the next genus in the direct line 

 of equine descent to Mesohippus is MinJdpp/'s. It is smaller in 



