skeleton between Eohippus and the recent horses, the former has stamped upon it an equine character which is unmistakable, though it can hardly be expressed in words. Each one of the different Eocene and Oligocene horizons has its characteristic genus of horses, showing a slow, steady progress in a definite direction, all parts of the structure participating in the advance. It is not necessary to follow each of these successive steps of change, but it should be emphasised that the changes are gradual and uninterrupted. The genus Mesohippus, of the middle Oligocene, may be selected as a kind of half-way stage in the long progression. Comparing Mesohippus with Eohippus, we observe that the former is much larger, some species attaining the size of a sheep, and has a relatively longer neck, longer limbs and much more elongate feet, which are tridactyl, and the middle toe is so enlarged that it bears most of the weight, while the lateral digits are very much more slender. The fore-arm bones have begun to co-ossify and the ulna is greatly reduced, while the fibula, though still complete, is hardly more than a thread of bone. The skull has a longer face and a nearly enclosed orbit, and the brain-case is fuller and more capacious, the internal cast of which shows that the brain was richly convoluted. The teeth are still very short-crowned, but the upper incisors plainly show the beginning of the "mark"; the premolars have assumed the molar form, and the upper molars, though plainly derived from those of Eohippus, have made a long stride toward the horse pattern, in that the separate cusps have united to form a continuous outer wall and two transverse crests. In the lower Miocene the interesting genus Desmatippus shows a further advance in the development of the teeth, which are beginning to assume the long-crowned shape, delaying the formation of roots; a thin layer of cement covers the crowns, and the transverse crests of the upper grinding teeth display an incipient degree of their modern complexity. This tooth-pattern is strictly intermediate between the recent type and the ancient type seen in Mesohippus and its predecessors. The upper Miocene genera, Protohippus and Hipparion are, to all intents and purposes, modern in character, but their smaller size, tridactyl feet and somewhat shorter-crowned teeth are reminiscences of their ancestry.