THE PERISSODACTYL UNGULATES 215 



f(or£),C^pf,M§.total34 (28). 



The broad upper incisor with its oblique chisel-shaped 

 edge shears against the lower lateral tusk-like incisor. The 

 premolars except the first are similar in appearance to the 

 molars though somewhat smaller. The upper molars show 

 obliquely placed lophs, not transverse as in the Tapirs, and 

 the ectoloph connecting the paracone and metacone is well 

 marked. Each lower molar exhibits a double crescent. 



The Horses, by which term we include also Asses, Zebras 

 and Quaggas, possess a dentition as complicated as that of 

 any existing Mammals and perhaps the best way to elucidate 

 the complexity of appearance in the teeth is to follow rapidly 

 the stages in evolution of the dentition from the first appear- 

 ance of the ancestral forerunner. 



In the little Eohippus or Dawn horse of Eocene times, an 

 animal scarcely larger than a cat, we find that the incisors and 

 canines are simple teeth and the premolars smaller and sim- 

 pler than the molars which show no evidence of the involved 

 pattern which they are later to assume. The upper molars 

 possess the three cusps of the trigon with the addition of a 

 hypocone and the first beginning of an ectoloph while the 

 transverse ridges which appear later in phylogeny are fore- 

 shadowed by the presence of a small cuspule between each pair 

 of cusps. The lower molars also are primitive teeth and, the 

 paraconid being lost, display two pairs of cusps united by low 

 transverse ridges. 



In the later Eocene the last premolar only had taken on 

 the molar pattern and the incisors were still simple cutting 

 teeth. Before the end of this period the third premolar in 

 addition had become molariform. 



Lower Oligocene Horses show all the premolars molariform 

 in appearance except the first; the upper incisors but not the 

 lower exhibit a low enamel ridge behind the cutting edge, the 

 first stage in the formation of what is now called the "mark." 



