THE PERISSODACTYL UNGULATES 217 



dentition it was stated that in the lower Oligocene the incisors 

 began to exhibit a low enamel ridge behind the cutting edge. 

 The development of this has resulted in the formation of a deep 

 enamel ridge behind the cutting edge. The development of this 

 has produced a deep enamel-lined pit, the 



Fig. 78. — Dentition of Zebra (Equus burchelli, 9.725-1). This old male shows 

 marked wearing of all the teeth. The upper canines are characteristic of the male. 

 For the elucidation of the cusp pattern of the molars consult the text. 



crown of the tooth, present in modern Horses in lower and 

 upper incisors alike. As the tooth wears down this tube-like 

 formation, since it never involves the entire crown, becomes 

 less deep and in the old animal (see Fig. 78) only the last 



