^^''"f ZOOGENESIS '^^^^ 



Gidley, gives the great depth to the jaws of modern 

 horses. In most of the horses of the later Tertiary 

 the cheek teeth are of this type, but the crowns are 

 less heightened and are usually much more curved. 

 Both of these features tend to decrease the depth of 

 the jaws. 



A few of the kinds of horses which lived in this 

 period had teeth of a simpler type in which the crowns 

 are low and are attached to the jaws by means of 

 roots. For the most part these kinds are found in the 

 earlier portions of the period. Of the kinds with 

 high crowned teeth those with the least heightened 

 crowns are also found in the earlier phases of this time 

 period. This group of the earlier horses represents 

 in a way a transition stage between those with low 

 crowned and those with high crowned teeth, for the 

 young or colts of the horses of this group had cheek 

 teeth of the low crowned type which in adult life 

 were replaced by teeth of the high crowned type. 



In the next older epoch of the Tertiary period, 

 known as the Oligocene, still other kinds of horses 

 inhabited America. These more ancient horses were 

 on the average smaller than those just noted. Most 

 of them were about the size of a shepherd dog or a 

 little larger; a few were somewhat smaller. There 

 were many different kinds of these little horses. All 

 were three-toed types with the lateral toes reaching 

 to the ground, and all had low crowned teeth, so 

 except for their smaller size they must have resembled 

 closely those of similar type belonging to the next 

 later period (Miocene). 



