THE FORERUNNERS OF THE HORSE 255 



surrounded by a complete ring of bone. It is of 

 special interest on account of the circumstance that 

 while its permanent cheek-teeth resemble those of 

 all the preceding genera in having their hollows 

 completely filled up with cement and the whole 

 crown relatively tall, those of the deciduous or milk 

 series are short-crowned, with their hollows open and 

 devoid of cement. The genus thus forms a grada- 

 tion in this respect from Protohippus to the lower 



Right Upper Milk-Molars (a) and Premolars (^) of the Extinct American 

 Mfiychippiis, | natural size 



and more generalised members of the horse-line. 

 The degree of complexity of the enamel-foldings 

 in the crowns of the upper cheek-teeth varies in the 

 different species. The feet are three-toed, with in 

 some instances a rudiment of a fourth, or outermost, 

 toe in the front pair ; that is to say, of a toe corre- 

 sponding with the human little finger. The two 

 lateral toes vary in size in the different species, but 

 in none did they touch the ground, so that the feet, 



