28o THE HORSE AND ITS RELATIVES 



again find both progression and retrogression, 

 as in the modern horse the canine and the first 

 premolar are alike reduced to vestiges and are 

 often entirely absent. The early horses had grinding 

 teeth of a very generalised pattern ; indeed, it is 

 often a matter of great difficulty to distinguish the 

 teeth of these horses from those of the ancestors of 

 what are now widely removed orders of mammals. 

 On their crowns these teeth bore little cusps or 

 prominences, which in the quadrangular molars just 

 begin to grow together into the crests that later 

 form the greater portion of the grinding surface. 

 The premolars are at first simple in character, but 

 as time goes on they become successively molar- 

 like, beginning with the hindermost. This is not 

 true of the anterior one, which, as we have seen, is 

 finally reduced to an often disappearing remnant. 



" During the forest-dwelling period in the 

 history of the horses, and while they lived upon 

 succulent meadow-grasses, the teeth, though in- 

 creasing in size with the entire organism, remain 

 short-crowned. Upon the expansion of the 

 prairies, however, and the adoption of the harsh 

 grasses as a main staple of food, the tooth of*the 

 horse changes in character, becoming elongate, 

 prismatic in shape, and the depression lying 

 between the crests filling with a substance known 

 as cement, which strengthens the entire tooth. 

 The result is a long columnar structure made up of 



