218 MAMMALIAN DENTITION 



remnant remains on the upper first and second incisors; upon 

 all others there is a dark stain, the dental ''star," an area of 

 secondary dentine formed just in front of the line of the 

 "mark." 



Turning next to the cheek teeth, we note that all the func- 

 tional premolars* are completely molariform in appearance, 

 and the talonid of the last, lower molar is elongated as usual in 

 herbivorous forms. But the crowns of all the cheek teeth show 

 an exceedingly complicated pattern. The teeth are hypsodont 

 as would be anticipated but show extreme specialization of the 

 occlusal surface. The crenated enamel ridges which in their 

 highly complex arrangement can be seen even in worn teeth to 

 be derived from the more primitive pattern of the teeth in 

 Eohippus by the addition of styles, lophs and cingula which, 

 if measured in all their closely packed infoldings, represent a 

 length of about sixteen inches, that is four times the circum- 

 ference of the tooth itself. Thus does Nature provide a surface 

 upon which slight ledges of enamel, projecting beyond the den- 

 tine of the lophs themselves and the cement which occupies the 

 spaces between the lophs, form roughenings always sharp and 

 ready for the mastication of the relatively tough and hard 

 grass stems upon which the animal feeds. It may seem a long 

 way from the comparatively simple teeth of Eohippus to the 

 intensely specialized teeth of the modern Horse with their 

 maze-like pattern but Ave know from fossils practically every 

 stage which intervened between them, and can assert pos - 

 tively that the complicated grinder of the modern Horse de- 

 veloped gradually in easy stages and by a slight modification 

 at a time during the vast geological period which has inter- 

 vened between the Eocene and the present day. 



It is worth while then to pause a moment and look back into 

 the paleontological history, not of the Horses alone, but of the 

 Tapirs and Ehinoceroses also. We have already noted the fact 

 that the Perissodactyla and Artiodactyla traced back to their 

 earliest known ancestors still shoAV such marked differences that 



f The "wolf" teeth of the specimen figured were lost many years before death. 



