EQUID^E 



377 





(Fig. 156, rt), although one species retained a rudiment of the 

 metacarpal of the pollex. 



The transition from these animals of the Eocene period to the 

 Horses of modern times has been accompanied by a gradual increase 

 in size. The diminutive Hymcotherium of the Lower, and Pachy- 

 nolophus of the Middle and Upper Eocene Avere succeeded in the 

 Miocene period by the forms to which the name of Anchitherium 

 has been given, of the size of sheep ; these again in Pliocene times 

 by Hipparion and Protohippus, as large as the modern donkeys ; and 

 it is mainly in the Pleistocene period that Equidce occur which 

 approach in size the existing Horse. Important structural modi- 

 fications have also taken place, with corresponding changes in the 



FIG. 156. Successive stages of modification of the feet of extinct forms of Horse -like 

 animals (chiefly from Marsh), showing gradual reduction of the outer and enlargement of the 

 middle toe (in), a, Pachynolophus (Eocene) ; b, Anchitherium (Early Miocene) ; c, Anchitherium 

 (Late Miocene) ; d, Ilipparion (Pliocene) ; e, Equus (Pleistocene). 



mode of life of the animal. Thus the neck has become elongated, 

 the skull altered in form, the teeth greatly modified, and the limbs 

 have undergone remarkable changes. The last two require to be 

 described more in detail. 



The teeth in the Eocene forms had, as mentioned above, the 

 characteristic number of forty-four. This number has been retained 

 throughout the series, at least theoretically ; but one tooth on either 

 side of each jaw, the anterior premolar, which in all the Eocene 

 and Miocene species was well developed, persisting through the 

 lifetime of the animal, is in all modern Horses rudimentary, 

 functionless, and generally lost at an early period of life, evidently 

 passing through a stage which must soon lead to its complete dis- 

 appearance. The canines have also greatly diminished in size, and 

 are rarely present in the female sex, so that practically a very large 

 number of adult Horses of the present day have eight teeth less 

 than the number possessed by their predecessors. The diastema 

 or interval between the incisor and premolar teeth (of essential 



