No representatives of the true horses are found in Europe in the Oligocene, and the single form, Ancliitherium, which occurs in the Middle and Upper Miocene, belongs to an aberrant group of forest animals which is also represented from the Middle Miocene to the Lower Pliocene in North America and in the Lower Pliocene of China. Only in the Lower Pliocene Pontian fauna from China to Portugal and in India, do we again meet with typical members of the horse family in individuals of the genus Hipparion. The living genus Equus, which is not a descen dant of Hipparion, appears suddenly in the Upper Pliocene of India and western Europe. South America received its horses in Pliocene times from North America, the peculiar genera of that continent having arisen from a North American Pliocene genusp The Tapiridae have a probable ancestor in the Lower Eocene of North America and unquestionably ancestral tapirs occur in the Oligocene of Europe and North America. The group appears to have survived in North America until Pleistocene times, reach ing South America after the two continents became connected in the Pliocene. In Europe and Japan a Lower or Middle Miocene form is known, and the living genus is represented in Upper Miocene and Lower, Middle and Upper Pliocene deposits. A gigantic form twice the size of the existing ones occurs in the Pleistocene of China. No representatives of this family appear to have been discovered in the Tertiary rocks of India, but it is clear that the existing Bornean form must have reached that region from the Palaearctic region.
The lophiodonts, a group allied to the tapirs but restricted to Eocene and Oligocene times, include cursorial forms, and occur not only in North America and Europe but also very abundantly in Mongolia.
The rhinoceroses present a very complicated evolutionary story not yet fully understood. One primitive group of animals, the amynodonts, somewhat hippopotamus-like in their adapta tions, occur in Eocene and Oligocene rocks in Europe, Mongolia, Burma and North America. The cursorial hydracodonts are known only from the Eocene and Oligocene of North America and Mongolia, whilst their associates of such genera as Dicerather ium occur both in Europe and North America. The evolution of the more normal rhinoceroses is still so little understood that it is impossible to discuss their migration; it is however clear that creatures essentially ancestral to the living Sumatran rhi noceroses occur widely spread in the Upper Pliocene and Pleisto cene of Europe, in association with forms allied to the African rhinoceroses. The living Indian rhinoceros has apparently been derived from forms present in that area in Pliocene times. One remarkable extinct rhinoceros, Elasmotherium, found in the Lower Pleistocene of Russia and Siberia, and very rarely in Germany, has an ancestor in the Pliocene of China.
The Titanotheriidae is a small family of perissodactyls whose first representatives are found at the top of the Lower Eocene of North America. They underwent a very rapid evolution in that
continent, soon attaining a gigantic size and a very highly special ised character. In the Middle Eocene of Transylvania a single lower jaw has been found. In the Upper Eocene of Mongolia typical members of American genera of similar age are to be found ; fragments of very nearly allied animals have been de scribed from Upper Eocene rocks in Burma, and in the Lower Oligocene of North America and Mongolia their remains are abundant. A few fragments have occurred in presumably Oli gocene rocks in Eastern Europe, but none have been found in the thoroughly explored deposits of this age in western Europe.
The only remaining group of perissodactyls, the chalicotheres, has a possible ancestor in the Middle Eocene of the United States. It then occurs in the Oligocene both of Europe and North Amer ica, is found in Middle Miocene times in India, and is a member of the Pontian fauna of southern Europe. It occurs in the Pleistocene of Central Africa after it had died out in other parts of the world.
The history of the Perissodactyla is of very great importance for the study of geographical distribution as so many fossil forms of the group are well known that relationships existing between the various members can be established with greater certainty than is the case in any other order of mammals. Further more many of its members were strange looking animals whose abundant and unmistakable fossil remains make the evidence with regard to its distribution during past times of peculiar reli ability. Judging solely from the perissodactyls, it would appear that in Lower Eocene times Europe, Mongolia and North Amer ica formed a continuous land-surface, presenting such a variety of vegetation as to allow of the free passage of forest as well as of plain animals. In Upper Eocene times Europe became partially separated from North America, so that certain groups, the palaeotheres and the titanotheres, which are swamp and forest forms, were unable to migrate freely from one region to the other. Horses however still occurred in very similar forms in the two regions. In Oligocene times this differentiation of fauna became exaggerated, the two areas becoming completely separated from one another. The intense earth movements of Miocene times so altered the geography of the world as to re-unite the Palaearctic and Nearctic regions and many forms are common to the two regions at this period. In the succeeding Lower Plio cene rhinoceroses are very abundant and varied in the Palaearctic and Oriental regions, whilst they are much more seldom found in America. Oligocene Africa had no perissodactyls but by mid Miocene times a rhinoceros had reached that continent. South America was colonized by horses and tapirs in Upper Pliocene times, buc no member of the group ever reached Notogaea.