In external appearance and general proportions the different families of existing perissodactyls have very little in common; that tapirs and rhinoceroses should be related is not surprising, but the horses would seem to be as far removed from both of the former as possible. Why, then, should they be included in the same order? A study of the skeleton, however, reveals the community of structure which obtains between the three families, a community which removes them widely from all other hoofed mammals. In all existing perissodactyls, though not in most of the Eocene genera, all the premolars, except the first, have the size and pattern of the molars. The foramina of the skull, or perforations by which blood-vessels and nerves enter and leave the cranium, are arranged in a way characteristic of the order and different from that seen in other hoofed mammals. The femur always has the third trochanter. The number of digits in each foot is usually odd, 1, 3 or 5, but four-toed forms occur, as the tapirs, which have four toes in the front foot, three in the hind; the important character is that the median plane of the foot bisects the third digit, which is symmetrical. The third and fourth, each asymmetrical, together form a symmetrical pair. Especially characteristic is the form of the astragalus and calcaneum (ankle and heel bones); the astragalus has but a single, deeply grooved and pulley-like surface, that for the tibia, the lower end is nearly flat and rests almost entirely upon the navicular, covering but little of the cuboid (see Figs. 146, 148). The calcaneum does not articulate with the fibula and its lower end is broad and covers most of the cuboid. While the foregoing list includes the most important of the structural features which are common to all perissodactyls and differentiate them from other hoofed animals, there are many others which it is needless to enumerate. [Illustration: FIG. 146.—Left pes of Tapir. _Cal._, calcaneum. _As._, astragalus. _N._, navicular. _Cn. 1_, _Cn. 2_, _Cn. 3_, first, second and third cuneiforms. _Mr. II_, _III_, _IV_, second, third and fourth metatarsals.] The subjoined table gives the families and principal genera of the American Perissodactyla; extinct groups are marked †. Suborder CHELODACTYLA. Normal Perissodactyls I. EQUIDÆ. Horses. _†Eohippus_, low. Eoc. _†Orohippus_, mid. Eoc. _†Epihippus_, up. Eoc. _†Mesohippus_, low. Oligo. _†Miohippus_, up. Oligo. _†Anchitherium_, up. Oligo. _†Parahippus_, low. Mioc. to low. Plioc. _†Desmatippus_, mid. Mioc. _†Hypohippus_, mid. Mioc. to low. Plioc. _†Merychippus_, mid. Mioc. to low. Plioc. _†Protohippus_, up. Mioc. _†Pliohippus_, up. Mioc. and low. Plioc. _†Neohipparion_, up. Mioc. and low. Plioc. _†Hipparion_, Plioc. _†Hippidion_, Pleist., S. Amer. _†Hyperhippidium_, Pleist., S. Am. _Equus_, Pleist., N. and S. Amer. II. †TITANOTHERIIDÆ. †Titanotheres.