384 AN INTRODUCTION TO THE STUDY OF FOSSILS 



by solid horny nails, the hoofs. (Hence the name from Latin 

 ungula, SL hoof.) Canine teeth absent or small ; premolars and 

 molars large, their broad crowns beset with ridges or tubercles. 



In the lowest Eocene it is almost impossible to distinguish the 



ancestors of mammals mth claws (the Unguiculata, including 

 orders 3-7) from those with hoofs (the Ungulata). They are 

 both flat-footed, five-toed animals with the toes terminating in 

 nails intermediate between claws and hoofs, with freely movable 

 fore limbs and tuberculate molar teeth, the tubercles in the for- 

 mer being slightly more pointed and cutting than in the latter. 

 The primitive Ungulata were mostly small, inhabiting marshes 

 or forests, vdth teeth adapted to succulent herbage. Many 

 branches of these early Ungulata (which were probably not 

 developed from any of the Condylarthra known at present) 

 became modified during the course of the Tertiary into hard- 

 hoofed dwellers of the dry, upland plains with teeth capable of 

 grinding the dry grasses. Grasses were probably established ar 



Fig. 165. — Restoration of skeleton of Phenacodus primcEvns, from the Lower 

 Eocene (Wasatch). This is one of the Condylarthra, — an exceedingly primitive 

 group of ungulates which serve to connect quite intimately the hoofed and clawed 

 mammals. (From Scott.) 



a rather dominant factor in the earth's vegetation by the time 

 of the Upper Eocene, though the siliceous grasses, now consti- 

 tuting the grasses of the plains, probably did not come into 

 prominence until the Miocene. As the Tertiary is ascended, 



