The now waning group of Perissodactyla would appear to have originally been a northern one, as all the three existing families, rhinoceroses (Rhinocerotidae), tapirs (Tapiridae), and horses (Equidae), are well represented in the Tertiaries of both halves of the northern hemisphere.
Finally, we have the family Rhinocerotidae, which includes the existing representatives of the group. In this family the dentition has undergone considerable reduction, and may be represented inclusive of all the variations, by the formula i a or a m a The first upper incisor, whenpresent, has an 430r2; PP antero-posteriorly elongated crown, but the second is small; when fully developed, the lower canine is a large forwardly directed tusk-like tooth with sharp cutting-edges, and biting against the first upper incisor.
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