The bones of the tarsus of mammals present fewer diversities of number and arrangement than those of the carpus. The proximal row (see Fig. 18) always consists of two bones, namely the astragalus (_a_), which probably represents the coalesced scaphoid and lunar of the hand, and the calcaneum (_c_). The former is placed more to the dorsal side of the foot than the latter, and almost exclusively furnishes the tarsal part of the tibio-tarsal or ankle-joint. The calcaneum, placed more to the ventral or “plantar” side of the foot, is elongated backwards to form a more or less prominent tuberosity, the “tuber calcis,” to which the tendon of the great extensor muscles of the foot is attached. The navicular bone (_n_) is interposed between the proximal and distal row on the inner or tibial side of the foot, but on the outer side the bones of the two rows come into contact. The distal row, when complete, consists of four bones, which, beginning on the inner side, are the three cuneiform bones, internal (_c¹_), middle (_c²_), and external (_c³_), articulated to the distal surface of the navicular, and the cuboid (_cb_), articulated with the calcaneum. Of these the middle cuneiform is usually the smallest in animals in which all five digits are developed; but when the hallux is wanting the internal cuneiform may be rudimentary or altogether absent. The three cuneiform bones support respectively the first, second, and third metatarsals, and the cuboid supports the fourth and fifth; they thus exactly correspond with the four bones of the distal row of the carpus. [Illustration: FIG. 18.—Bones of the right Human foot. _T_, Tarsus; _M_, metatarsus; _Ph_, phalanges, _c_, calcaneum; _a_, astragalus; _cb_, cuboid; _n_, navicular; _c¹_, internal cuneiform; _c²_, middle cuneiform; _c³_, external cuneiform. The digits are indicated by Roman numerals, counting from the tibial to the fibular side.] In addition to these constant tarsal bones, there may be supplemental or sesamoid bones: one situated near the middle of the tibial side of the tarsus, largely developed in many Carnivora and Rodentia; another, less frequent, on the fibular side; and a third, often developed in the tendons of the plantar surface of the tarsus, is especially large in Armadillos. There is also usually a pair of sesamoid bones on the plantar aspect of each metatarso-phalangeal articulation. In the young of the carnivorous genus _Crytoprocta_ there may be a second centrale, which usually coalesces with the ectocuneiform.