EQUIDAE B77. 
(Fig. 156, a), 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 Hyracotherium of the Lower, and Pachy- 
nolophus of the Middle and Upper Eocene were 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 Equide occur which 
approach in size the existing Horse. Important structural modi- 
fications have also taken place, with corresponding changes in the 
Fic. 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 (111). a, Pachynolophus (Eocene) ; b, Anchitheriwm (Early Miocene) ; c, Anchitherium 
(Late Miocene); , Hipparion (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 
