[Illustration: FIG. 187.—Left manus of Pig. _S._, scaphoid. _L._, lunar. _Py._, pyramidal. _Pis._, pisiform. _Td._, trapezoid. _M._, magnum. _U._, unciform. _Mc. I_, second, _Mc. II_, third, _Mc. III_, fourth, _Mc. IV_, fifth, metacarpals.] [Illustration: FIG. 188.—Left pes of Pig. _Cal._, calcaneum. _As._, astragalus. _N._, navicular. _Cb._, cuboid. _Cn. 2_, _Cn. 3_, second and third cuneiforms. _Mt. II-V_, second to fifth metatarsals.] [Illustration: FIG. 189.—Bunodont upper molar of peccary (_Tagassu_).] [Illustration: FIG. 190.—Selenodont upper molar of deer (_Odocoileus_).] Especially characteristic of the order is the structure of the ankle, or “hock-joint” of the hind limb. The ankle-bone, or astragalus, has a double pulley, the upper and lower ends being of quite similar shape; its lower end is almost equally divided between the cuboid and navicular, which are made concave to receive it. This type of astragalus is altogether peculiar to the artiodactyls, all of which possess it; it is unlike that of any other mammal whatever and may be recognized at a glance. The calcaneum, or heel-bone, has a large convex facet, by means of which it articulates with the fibula, or external leg-bone; there is no such articulation in the perissodactyls. The lower end of the calcaneum is narrow and fits into a step cut in the cuboid, which is thus every whit as peculiar and characteristic as the calcaneum and astragalus. The femur never has the third trochanter, which is always present in the perissodactyls. Another respect in which the artiodactyls differ from all perissodactyls except the horses is in the much more complex mode of articulation between the vertebræ of the lumbar and posterior dorsal regions, which the former display, and even the horses have no such elaborate arrangement. Finally, another very marked difference from the perissodactyls is in the teeth, for the premolars and molars are never alike, and only in very rare instances does the last premolar assume the molar-pattern. Of this pattern, there are two principal kinds, one exemplified by the peccaries, in which the crown supports a series, fundamentally two pairs, of conical cusps, and called _bunodont_, and the other, to be seen in all the ruminating animals, in which the crown is composed of two pairs of crescents and is therefore said to be _selenodont_. The bunodont was the primitive type, whence the other was derived, and many transitional forms are known. The classification of the immense horde of living and extinct genera and species which are referable to the artiodactyls is an extremely difficult problem, which has found no thoroughly satisfactory solution and will not until much more is learned concerning the history of the order and conflicting opinions can be reconciled. The most important American families and genera are given below, though the arrangement is but tentative. Suborder A. ARTIODACTYLA †PRIMITIVA. (Extinct genera of doubtful affinities)