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 Hoofed Mammals.—All the animals in this order walk on the tips of their toes, which have been adapted to this use by the claws having developed into hoofs. The order is subdivided into the odd-toed (such as the horse with one toe and the rhinoceros with three) and the even-toed (as the ox with two toes and the pig with four). All the even-toed forms except the pig and hippopotamus chew the cud and are given the name of ruminants.

Horse and Man Compared (Figs. 381, 399).—To which finger and toe on man's hand and foot does the toe of a horse's foot correspond? Has the horse kneecaps? Is its heel bone large or small? Is the fetlock on toe, instep, or ankle? Does the part of a horse's hind leg that is most elongated correspond to the thigh, calf, or foot in man? On the fore leg, is the elongated part the upper arm, forearm, or hand? Does the most elongated part of the fore foot correspond to the finger, palm, or wrist? On the hind foot is it toe, instep, or ankle? Is the fetlock at the toe, instep, or heel? (Fig. 385.) Is the hock at the toe, instep, heel, or knee? Order. Why?.

—Left leg of man, left hind leg of dog and horse; homologous parts lettered alike.

Specializations of the Mammals.—The early mammals, of which the present marsupials are believed to be typical, had five toes provided with claws. They were not very rapid in motion nor dangerous in fight, and probably ate both animal and vegetable food.