. An introduction to the study of mammals living and extinct. Mammals. 38o UNGULATA and endurance surpassing that of almost any other animal. When surprised, however, they are by no means helpless, both fore and hind feet becoming at need powerful weapons of defence. If we were not so habituated to the sight of the Horse as hardly ever to consider its structure, we should greatly marvel at being told of a mammal so strangely constructed that it had but a single toe on each extremity, on the end of the nail of which it walked or galloped. Such a conformation is without a parallel in the vertebr
. An introduction to the study of mammals living and extinct. Mammals. 38o UNGULATA and endurance surpassing that of almost any other animal. When surprised, however, they are by no means helpless, both fore and hind feet becoming at need powerful weapons of defence. If we were not so habituated to the sight of the Horse as hardly ever to consider its structure, we should greatly marvel at being told of a mammal so strangely constructed that it had but a single toe on each extremity, on the end of the nail of which it walked or galloped. Such a conformation is without a parallel in the vertebrate series, and is one of the most remarkable instances of specialisation, or deviation from the usual type, in accordance with particular conditions of life. It is clear, both from the structure of the foot itself, and also by an examination of the intermediate forms, that this toe corresponds to the middle or third digit of the complete typical or pentadactyle foot; and there is very strong evidence to show that by a gradual concentration of all the power of the limb upon this toe, and the concurrent dwindling away and final dis- appearance of all the others, the present condition of the Horse's foot has been produced. FrotoMppus.^—In this Lower Pliocene North American genus (also described as Merychippus) the cheek-teeth resemble those of the generalised species of Eguus, but have shorter crowns; while the milk-molars approximate to the permanent molars of Anchi- therium. Each foot has three digits. Hipparion.^—Upper cheek-teeth (Fig. 159), with the antero-. c f d i Fig. 169.—Three right upper cheek-teeth of Hippanon. a, Antero-external column; b, postero-external column ; o, postero-internal column, or posterior pillar; d, antero-internal column, or anterior pillar; /, posterior intermediate column; i, anterior intermediate column. (From the Palceontologia Indica,) internal column, or anterior pillar as it may be conveniently. termed in this family, detached throughou
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