The Perissodactyla may be divided into the four following sections, namely the extinct Titanotheroidea, the Hippoidea, represented by the horse tribe and their ancestors, the Tapiroidea, typified by the tapirs, and the Rhinocerotoidea, which includes the modern rhinoceroses and their forerunners.
Rhinoceros Group. - The last section of the Perissodactyla is that of the Rhinocerotoidea, represented by the modern rhinoceroses and their extinct allies.
In North America rhinoceroses became extinct before the close of the Pliocene period; but in the Old World, although their geographical distribution has become greatly restricted, at least five well-marked species survive.
Hornless rhinoceroses, with five fronttoes, ranging from the Oligocene to the Lower Pliocene in Europe, represent the genus Aceratherium, which may also occur in America, as it certainly does in India.
With the short-skulled, short-footed, three-toed and generally horned rhinoceroses ranging in Europe and America from the Lower Miocene to the Lower Pliocene, typified by the European R.
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