216 MAMMALIAN DENTITION 



The molars were still brachyodont. It must be remembered 

 that in the Eocene the climate of Europe and North America 

 was moist and subtropical and only during later ages did the 

 elevation of the land and the consequently colder climate re- 

 sult in the formation of grassy plains. In the Miocene the 

 forested swamp had given place in many districts to grassy 

 plains and it is therefore in this period that we look for the 

 development of high-crowned grazing molars instead of the 

 brachyodont broAvsing cheek teeth universal among Mammals 

 in earlier ages. In the middle Miocene we first find Horses 

 possessing high-crowned teeth with coverings of cement such 

 as are found in the Horses of today, but the milk dentition, 

 lagging in progress behind the permanent teeth, still exhibits 

 brachyodont molars. For some time during the Pliocene 

 browsing Horses existed side by side with the more advanced 

 grazing type but gradually became extinct and gave place to 

 the latter more adapted form in which, however, the molars 

 though distinctly foreshadoAving the pattern exhibited by 

 modern Horses, had by no means reached the present degree 

 of hypsodonty. 



The modern Horse (Fig. 78) possesses teeth which are ex- 

 tremely high crowned and have roots growing continuously 

 until the animal is quite old. The dental formula is: 



if. C7,P'7,m|. total 44. 



In consequence of the length of the teeth the orbit has 

 been pushed back beyond the tooth sockets and this results in 

 a very elongated facial region. The first premolar, the 

 "wolf" tooth, of each jaAv is very small and functionless 

 being lost early and the upper canines occur only in the 

 male. The incisors are all stout teeth, the lowers someAAdiat 

 procumbent, and meet in an edge to edge bite which, admi- 

 rable for cropping grass, results in great and continuous Avear- 

 ing of the croAvn. In surveying the evolution of the equine 



