Comparison of animals' foot anatomy. Coloured historical artwork of the bones of the foot of an elephant, hippopotamus, rhinoceros, ox and horse. The number of weight-bearing toes reduces from left to right. In the elephant, the inner toe (marked I) is reduced and contains fewer bones than the others. This toe is seen to be greatly reduced or absent in the feet of the other animals. The number of toes is a characteristic feature of two mammal orders; the Artiodactlya (even-toed ungulates such as the hippo, ox, pig and deer) and Perissodactlya (odd-toed ungulates such as the rhino, horse and tapir). This artwork was published in The Human Foot and The Human Hand by G M Humphry, in 1861.