. An introduction to the study of mammals living and extinct. Mammals. EQUID^ 377 although one species retained a rudiment of the (Fig. 156, ci), metacarpal of the poUex. 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 Hyracoiherium of the Lower, and Pachy- iiolophus of the Middle and Upper Eocene were succeeded in the Miocene period by the forms to which the name of Anehitherium has been given, of the size of sheep; these again in Pliocene times by Hipparion and Protohippus, as large as the
. An introduction to the study of mammals living and extinct. Mammals. EQUID^ 377 although one species retained a rudiment of the (Fig. 156, ci), metacarpal of the poUex. 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 Hyracoiherium of the Lower, and Pachy- iiolophus of the Middle and Upper Eocene were succeeded in the Miocene period by the forms to which the name of Anehitherium 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 (iii). a, PacJiynoloijlitis (Eocene) ; 6, Anehitherium (Early Miocene) ; c, Aiichitheriwni (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 th
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