32 THE HORSE. 



still existing, the horse alone having advanced to a 

 still more specialized state. 



The teeth of Hyracotherium and its allies are the 

 same in number as in Phenacodus — the same, in 

 fact, as in the vast majority of Eocene mammals j 

 but they begin to show more distinctly, especially in 

 the grinding surfaces of the molars, a pattern in 

 which the groundwork of all the subsequent com- 

 plex modifications can be clearly traced (Fig. 3, B). 

 The four larger corner cusps are distinct, but the 

 intermediate ones are assuming the form of ridges 

 or crests connecting the two anterior and the two 

 posterior cusps respectively. These ridges are of a 

 curved or sinuous form, and are not placed quite 

 transversely, but have their inner ends inclining 

 backwards. It will be usefid to become thoroughly 

 acquainted with this pattern, as it is the key to all 

 others which will be hereafter spoken of. 



In deposits of corresponding and more recent age 

 to those in which the remains of Hyracotherium 

 were found, immense numbers of bones and teeth 

 have been discovered, indicating a variety of spe- 

 cies more or less diverging in details, although con- 

 structed in the main upon the same type, the best 

 known of which are included in the genera Lo~ 

 phiodon of European and Hyrachyus of American 

 formations. Of the latter, remarkably complete 



