The Perissodactyla have been brigaded with the Artiodactyla to form the typical group of the ungulates, under the name of Diplarthra, or Ungulata Vera, and the features distinguishing the combined group from the less specialized members of the order Ungulata will be found under the heading of that order.
The following are the leading characteristics by means of which the sub-order Perissodactyla is distinguished from the Artiodactyla.
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.