30 



THE HORSE 



show more distinctly, especially in the grinding surfaces 

 of the molars, a pattern in which the groundwork of all 

 the subsequent complex 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 



Fig. 3.— Grinding surface of upper molar tooth (very 

 slightly worn). A, Phenacodus. B, Hyracotherium. C, 

 Anchitherium . a, antero-external cusp ; b, antero- 

 internal cusp ; £,postero-externalcusp; d, postero-internal 

 cusp. 



transversely, but have their inner ends inclining back- 

 wards. It will be useful 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 dis- 

 covered, indicating a variety of species more or less 



A 



B 



C 



