The section is divisible into the families Equidae and Palaeotheriidae, of which the latter is extinct.
In the Equidae the premolars are generally or.
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.
1-9 from "American Equidae.") Eohippus sp. Eohippus sp.
Callosities, or bare patches covered with hardened and thickened epidermis, are found on the buttocks of many apes, the breast of camels, the inner side of the limbs of Equidae, the grasping under-surface of the tail of prehensile-tailed monkeys, opossums; &c. The greater part of the skin of the onehorned Asiatic rhinoceros is immensely thickened and stiffened by an increase of the tissue of both the skin and epidermis, constituting the well-known jointed " armour-plated " hide of those animals.
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.
Species The distinctive characteristics of the family, and its position in the zoological system, are given in the articles Equidae and Perissodactyla.
Small round or oval plates of horny epithelium called " chestnuts," callosities growing like the hoof from enlarged papillae of the skin, are found on the inner face of the fore-arm, above the carpal joint in all species of Equidae, and in the horse (E.
Xx., 1897, and " A New Arrangement of the Existing Species of Equidae," Op. cit.
According to Ridgeway, the original source of the finest equine blood is Africa, still the home of the largest variety of wild Equidae; he concludes that thence it passed into Europe at an early time, to be blended with that of the indigenous Celtic species, and thence into western Asia into the veins of an indigenous Mongolian species, still represented by " Przewalski's horse "; not till a comparatively late period did it reach Arabia, though the " Arab " now represents the purest form of the Libyan blood.
The foregoing genera constitute the subfamily Equinae, or the Equidae as restricted by the older writers.
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.