Finally, the laws of distribution of animals over Siberia cannot be made out until the changes undergone by its surface during the Glacial and Lacustrine periods are well established and the Post-Tertiary fauna is better known The remarkable finds of Quaternary mammals about Omsk and their importance for the history of the Equidae are merely a slight indication of what may be expected in this field.
Species The distinctive characteristics of the family, and its position in the zoological system, are given in the articles Equidae and Perissodactyla.
The section is divisible into the families Equidae and Palaeotheriidae, of which the latter is extinct.
They have all the great peculiarity, not found in the teeth of any other mammal, and only in the Equidae of comparatively recent geological periods (see also Palaeontology), of an involution of the external surface of the tooth (see fig.
Mesohippus and Miohippus are connected with the earliest and most primitive mammal which it is possible to include in the family Equidae by means of Epihippus of the Uinta or Upper Eocene of North America, and Pachynolophus, or Orohippus, of the Middle and Lower Eocene of both halves of the northern hemisphere.
The foregoing genera constitute the subfamily Equinae, or the Equidae as restricted by the older writers.
In the Equidae the premolars are generally or.