RELATION OF MAN" TO THE TERTIARY MAMMALIA. 273 



In the heel-bone we have a succession from the short and flat 

 form of Coryphodon to the long and slender one of the horse and 

 ruminants ; the increase in length being associated with the elon- 

 gation of the bones of the toes, and the assumption of the digiti- 

 grade type from the planti- 

 grade. The mammals of the 

 Lower Eocene* exhibit a 

 greater percentage of types 

 that walk on the entire sole 

 of the foot, while the suc- 

 ceeding periods exhibit an 

 increasing number of those 

 that walk on the toes, while 

 the hoofed animals and Car- 

 nivora of recent times nearly 

 all have the heel high in the 

 air, the principal exceptions 

 being the elephant and the 

 bear families. 



A most noticeable succes- 

 sion is seen in the diminu- 

 tion of the number of toes. 

 In the series leading to the 

 horse, the ox, and the hyaena 

 and cat, this reduction pro- 

 ceeds by the loss of a toe 

 from the one side or the 

 other, until in the ruminants 

 but two are left, and in the 

 horse but one. The series ex- 

 tending from the primitive 

 Eocene types with five digits, 

 to the existing reduced forms, 



is most complete, although Fig. b\.—Phenacodus primcevus skull, one 



a few of the New Mexican third natural si^e, from below, showing quadri- 

 -c^ ,1 1 tubercular true molars. From specimen figured 



Eocene genera themselves , . r & 



^ _ on plate. 



probably exhibit but four 



digits on one or both pairs of feet. The presence of the rudiments 



* All the Mammalia of the Puerco fauna (which was unknown at the time this 

 lecture was delivered) are plantigrade. (Ed. 1886.) 



18 



