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
Fig. 124.—Side view of skull of Horse with the bone removed so as to expose the whole of the teeth. c, Canine; Fr, frontal; i1, i2, i3, incisors; L, lachrymal; m1, m2, m3, molars; Ma, malar or jugal; Mx, maxilla; Na, nasal; oc, occipital condyle; Pa, parietal; pm1, situation of the vestigial first premolar, which has been lost in the lower, but is present in the upper jaw; pm2, pm3, pm4, remaining premolars; PMx, premaxilla; pp, paroccipital process; Sq, squamosal. (After Flower and Lydekker.)
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
