touch, the ground, shows very clearly the position assumed by the bones of the foot in these animals. Not only are the heel and the first long joints raised above the ground, as in the dogs and cats, but the two remaining long upper joints join together to form one strong bone (Fig. 4 *), and none of the lower joints of the toes touch, the ground. The nails of two toes which form what we call the "hoof" rest upon it, while at a we see all that is left of a third and fourth toe, possessed by the distant ancestors of the bison.
The chief distinction between the feet of the different nail-walkers is the number of toes whose nails form the hoof. Fig. 5 A shows us that the pig has four toes, but only two touch the ground and make the hoof; the other two are useless, and are gradually becoming smaller. The rhinoceros (Fig. 5 B) has only three toes, and these are not all equal in length, but all are in use and end in massive hoofs. The cow (Fig. 5 C) has only two toes, the upper joints of which have grown together into one as in Fig. 4 (for the bison is one of the cows) so as to form a long, strong part of the leg, the two very
thick nails making a double hoof. This is what is called in the Bible a "cloven" hoof; it is not, however, as was supposed, a single nail split down, but two distinct nails belonging to two toes. Then, lastly, the horse (Fig. 5 D) has lost all its toes except one, which has become exceedingly thick and strong. The horse on its four feet, each, ending in only one toe with its great nail, attains immense speed in running, great length being given to its stride by the length-