Beginning with the Perissodactyla, or odd-toed ungulates, we find the Equine animals remarkably numerous and interesting. The true horses of the genus _Equus_, so abundant in the Post-Pliocene formations, are represented in the Pliocene by several ancestral forms. The most nearly allied to _Equus_ is _Pliohippus_, consisting of animals about the size of an ass, with the lateral toes not externally developed, but with some differences of dentition. Next come _Protohippus_ and _Hipparion_, in which the lateral toes are developed but are small and functionless. Then we have the allied genera, _Anchippus_, _Merychippus_, and _Hyohippus_, related to the European _Hippotherium_, which were all still smaller animals, _Protohippus_ being only 2½ feet high. In the older deposits we come to a series of forms, still unmistakably equine, but with three or more toes used for locomotion and with numerous differentiations in form, proportions, and dentition. These constitute the family Anchitheridæ. In the Miocene we have the genera _Anchitherium_ (found also in the European Miocene), _Miohippus_ and _Mesohippus_, all with three toes on each foot, and about the size of a sheep or large goat. In the Eocene of {136}Utah and Wyoming, we get a step further back, several species having been discovered about the size of a fox with four toes in front and three behind. These form the genus _Orohippus_, and are the oldest ancestral horse known. Prof. Marsh points out the remarkably perfect series of forms in America, which, beginning with this minute ancient type, is gradually modified by gaining increased size, increased speed by concentration of the limb-bones, elongation of the head and neck, the canine teeth decreased in size, the molars becoming longer and being coated with cement--till we at last come to animals hardly distinguishable, specifically, from the living horse.