636 



THE EXTERIOR OF THE HORSE. 



attached to the anterior branch of the 53 by a narrow pedicle of ivory. In this 

 respect there is no exception in the horse. In the 

 hipparion, on the contrary, which some evolutionists 

 consider as the ancestor of the preceding, this loop 

 is constantly isolated on the dental table (Fig. 306). 



Numerous differences concerning the disposition 

 of the folding of the enamel are also found. They 

 form at times, in certain animals, remarkable wavy 

 or sinuous folds, analogous to those on the tooth of 

 the hipparion. Some anatomists have considered 

 these characteristics as sufficient reasons for the es- 

 tablishment, among equine fossils, of an intermediary 

 species between the hipparion and our actual horse, 

 the Equus plicidens (plio-hippus). Whether this dis- 

 tinction is justified or not, the truth is that these 

 enamel plications still exist in many horses of to-day. 



In the inferior molars, the configuration of the 

 surface of friction differs slightly from that of the 

 superior ; Fig. 307 gives a sufficiently accurate idea 

 of this. 



Let us recall, in conclusion, that the three sub- 

 stances, enamel, dentine, and cement, are not equally 





FIG. 306 Superior left molar 

 arcade of the hipparion. 



FIG. 307. Transverse section of an inferior right molar 

 (enlarged). 



