EVOLUTION OF THE HORSE 



Upper Miocene. In this stage the crowns of the upper molars 

 have become much Longer, the two pairs of crescents on the upper 

 molars are complete, with two half-separated cusps within the 

 inner pair. And the valleys between the crests have become 

 filled with cement, so that with the wear of the teeth the edg< 

 hard enamel are backed inside by dentine and outside by cement. 

 In this way the surface of the tooth has a series of enamel ridges 

 always projecting a little above the grinding surface, because the 

 •r material on each side wears down into hollows, yet m 



. because they are braced so thoroughly on each side. 

 This is a very efficient instrument for grinding hard grasses. In 



tohippus and Pliohippus, especially in the former, the crowns 

 of the teeth are by no means as Long as in the modern h< »r 

 they must therefore wear more slowly or wear out at an earlier 

 age. 



The feet in these two genera have but one toe touching the 

 ground. The side toes (second and fourth digits) are complete, 

 but much more slender than in the earlier stages and are ap- 

 ntly useless, as they cannot reach the ground. In some 

 species of PlioJiippus they have almost disappeared. The fore- 

 foot of Protohippus still retains tiny nodules of bone at the back 

 of the "wrist" (sometimes improperly called in the Horse the 

 "knee-joint"), which are the remains of the first and fifth digits. 



Hipparion. PLIOCENE. This genus, probably also a side 

 branch of the genealogical tree of the horse family, is much like 



I 'hippits, but larger and with more complication about the 



h pattern. It is common in the European Pliocene beds and 

 n found in America also. Ti -till three-toed, 



the side t< >es as large as th< se I >f the i >lder ProtohippUS. 



12. Equus. Pleistocene and Recent. In this si 

 that -of the modern II rse, the side toes have entirely disappeared 

 and plints on the fore- and hind-foot. No 



• the little m dules which in Pi 



hippus repn I and fifth digits. The crown* 



b are much longer than in I -id of the two half- 



1 inner columi □ the upper molar has <li 



ed in size and changed in form. 



The skull has lengtl rid the animal is much lar. 



