As changes in the structure of the feet progressed, the central toe of the original five continued to increase in size while the adjacent digits became relatively shorter and eventually so reduced in length that they could touch the ground no longer. The smaller bones at the extremities, corresponding to the joints of our fingers and toes, eventually disappeared from the side toes. Then the longer bones of the outer digits lost the broadened supporting surface, where the missing toes had been attached, and became reduced to pointed remnants known as splints. Extreme shortening of the splint bones eventually leaves only a small knob which is often referred to as a rudimentary toe. In the skeleton of a large horse the splints are readily seen, but in some of the earlier species they are so small that they may easily be destroyed or overlooked by the collector who removes the fossilized material from the surrounding rocks. Even then, the bones of the wrist and ankle may indicate in an unmistakable manner that an additional toe once was present, for each bone is supported by another, and at the point of attachment there is a characteristic surface whose purpose is usually obvious. Throughout the Cenozoic era the changes continued. Among the horses of the North American Oligocene were _Mesohippus_, approximately the size of a collie dog, and _Miohippus_ which was slightly larger. Both were three-toed, but the rudimentary splint of a fourth toe was still present in the front foot. _Parahippus_ and _Merychippus_ carried on during the Miocene period, the latter being characteristic of the time, and showing, in addition to other progress, a decided trend toward the modern structure of molar teeth. There was some increase in size but the largest horse of that period was hardly more than a small pony. [Illustration: A Pleistocene Horse of the Texas Plains (_Equus scotti_)] _Hipparion_ and _Protohippus_, living during Upper Miocene and Pliocene time, represent later stages of the three-toed condition. The side toes were completely formed but greatly shortened, only the central toe touching the ground. In some of the species the outer toes had also become very slender, approaching the splint condition. By this time the molar teeth were longer and better adapted for feeding on grasses which were becoming sufficiently abundant to attract some of the forest dwellers into the open country. During the Pliocene period, in the genus _Pliohippus_ and also in _Hipparion_, the feet were far advanced in structure, with most of the species single-toed, the side digits having reached the splint stage. Pleistocene horses of the genus _Equus_, like living species of that genus, were strictly one-toed animals, ranging over grassy areas and highly specialized for a life in that kind of environment.