The artiodactyls are a group of hoofed mammals set off from the others primarily by the tendency to reduce the foot to the third and fourth digits. The earlier forms may have four toes, occasionally a trace of the fifth, but in all cases the weight falls between the third and fourth digits, so that they are equally developed, and there is no possible tendency to develop a single-toed foot, as in the perissodactyls. At its maximum development this makes the “cloven hoof” which is never a cloven hoof, but the incomplete fusion of the two above-mentioned digits. This group is also characterized throughout by the astragalus bone in the ankle having a roller surface on its lower end as well as the usual roller surface on the upper end—an astragalus which is unique. The group includes a wide range of animals, some but little specialized, like the . . .