Edentata. In the Sloths the leg bones are all long and slender. The femur has no third trochanter, and the fibula is complete and nearly equal in size to the tibia. In the Megatheriidae the leg bones are extraordinarily massive, the circumference of the shaft of the femur in Megatherium equalling or exceeding the length of the bone. There is no third trochanter in Megatherium. In most of the remaining Edentata the leg bones are strongly developed. The femur in the Armadillos and Aard Varks has a strong third trochanter, and the tibia and fibula are both large and are commonly ankylosed together at either end. The limb bones are very massive also in the Glyptodonts.

Sirenia. In no living Sirenian is there any trace of a hind limb, but in Halitherium a vestigial femur is found, which articulates with the pelvis by a definite acetabulum.

Fig. 109. Left femur of an Ox (Bos taurus) (to the left) and of

a Sumatran Rhinoceros (R. sumatrensis) (to the right). × 1/6.

(Camb. Mus.)

1. head.4. third trochanter.
2. great trochanter.5. shaft.
3. lesser trochanter.6. condyles.

In the Mystacoceti among the Cetacea small nodules of bone or cartilage occur connected with the vestigial pelvis, and may represent the femur and tibia. No trace of the skeleton of the hind limb is known in the Odontoceti.

In the Ungulata vera the femur is noticeable for the size of the great trochanter (fig. 109, 2); there is no definitely constricted neck separating the head from the rest of the bone, and the lesser trochanter (fig. 109, 3) is not very prominent. All Perissodactyles except the Chalicotheriidae show a strongly marked third trochanter, but this is absent in all known Artiodactyles. The development of the fibula in general corresponds to that of the ulna. In Rhinoceros, Macrauchenia, Tapirus and the Suina it is distinct and fairly well developed; in the Tragulina on the other hand it is vestigial, being reduced to the proximal end only. In the Ruminantia and Tylopoda also, it is much reduced forming merely a small bone attached to the distal end of the tibia, sometimes, as in the Red deer a slender vestige of the proximal end also is preserved quite detached from the distal portion; in the Horse this proximal portion is all that there is found of the fibula. The progressive diminution of the fibula can be well seen in the series of forms that are regarded as the ancestors of the Horse. The patella of the Ungulata vera is well ossified, but fabellae[177] are not usually found.

Subungulata. Of the Toxodontia, Toxodon has no third trochanter while Typotherium and Astrapotherium have one. In the Condylarthra the femur has well-marked lesser and third trochanters, and the fibula and patella are well developed. In the Hyracoidea there is a slight ridge on the femur in the place of the third trochanter, the fibula is complete, but is generally fused to the tibia at its proximal end.