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.
In the Hippoidea there is generally the full series of 44 teeth, but the first premolar, which is always small, is often deciduous or even absent in the lower or in both jaws.
The incisors are chisel-shaped; and (unlike the early Hippoidea) there is no gap between the first premolar, when present, and the second.
The lower cheekteeth have, unlike those of the Tapiroidea, crescentic ridges, which have not the loops at their extremities characteristic of the advanced Hippoidea; the last lower molar has no third lobe.
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