FAM. 1. EQUIDAE.--This family, which includes the living Horse, Zebras, and Asses, as well as a number of extinct genera agreeing with those types in structure, may be defined by the possession of but one functional toe, the two lateral ones being mere splints, or but little more. The molar teeth are hypselodont, and {238} the premolars, with the exception of the first, resemble the molars in their pattern. The orbit is completely surrounded by bone. The incisors are chisel-shaped, with a pit on the free surface. The canines are rudimentary if present. The radius and ulna are fused, as are the tibia and fibula. Although for the sake of uniformity a family, Equidae, is here separated from its allies, it is quite impossible owing to the full state of our knowledge of this group to draw a really hard-and-fast line between this family and the Palaeotheriidae. We shall deal presently with the conjectured pedigree of the Horse, which naturally involves that family, and which presents an unbroken series from four-toed Perissodactyles to the present one-toed Horse, the various bones and teeth becoming modified in the course of the descent "with the regularity of clockwork." We are compelled to draw the line at functional second and third toes; directly these are no longer used the animal is a Horse in the strict sense! This is irrational and regrettable, but necessary for practical purposes, if {239} we are to continue the plan of defining the various families of Mammalia. The genus _Equus_[150] contains not only the Horse, but the Asses and Zebras. The genus is to be distinguished as regards external characters by the following features:--The body is thickly clothed with hair; there is a more or less bushy tail and mane; the colours are apt to be disposed in stripes of black or blackish upon a yellowish brown ground; this is of course best seen in the Zebras, but the wild Asses also have some traces of it, if only in the single cross-bar of the African Wild Ass, and it is even "reversionary" in the domestic Horse at times. There are no horns upon the forehead or elsewhere; the fore-limbs or both pairs have a callous pad upon the inside, which is possibly to be looked upon as an aborted gland, possibly originally of use as secreting some odorous substance calculated to enable strayed members of the herd to regain their companions. The terminal phalanx of each of the (functionally) single digits is enclosed in a large horny hoof.