[Illustration FIG. 65. PELVIC GIRDLE AND SACRUM OF A. CASSOWARY (_Casuarius galeatus_) × 1/8. B. OWEN'S APTERYX (_A. oweni_) × 1/2. C. BROAD BILLED RHEA (_R. macrorhyncha_) × 1/6. D. OSTRICH (_Struthio camelus_) × 1/10. (All Camb. Mus.) 1. ilium. 2. ischium. 3. pubis. 4. acetabulum. 5. pectineal process.] POSTERIOR LIMB. The tibia is always well developed and has a very strong cnemial crest. The proximal tarsals are fused with its distal end, the whole forming a compound bone, the tibio-tarsus. There is frequently an oblique bar of bone crossing the anterior face of the tibio-tarsus at the distal end, just above the articular surface of the tarso-metatarsus, this is absent in Ostriches and _Æpyornis_. The fibula though in the embryo and in _Archaeopteryx_ equal in length to the tibia, is in the adult of other birds always imperfect, its proximal end is often fused with the tibia, and its distal end is commonly atrophied. In the Penguins however the distal end is complete. The distal tarsals fuse with the second, third and fourth metatarsals, forming a compound bone, the tarso-metatarsus. The first metatarsal is nearly always free but occasionally as in _Phaëthon_ it is fused with the others. No adult bird has more than four digits in the pes. In the Penguins the metatarsals are separate, and in many birds larger or smaller gaps exist between the fused metatarsals. In most birds the third metatarsal is curved so as not to lie in the same plane as the others, but in the Penguins they all three lie in the same plane. The metatarsals are clearly separated in _Archaeopteryx_. In Gallinaceous birds the tarso-metatarsus bears a bony outgrowth which is sheathed in horn and forms a spur. In most birds the first four toes are present while the fifth is always absent. The first toe commonly has two phalanges, the second three, the third four, and the fourth five. In Swifts the third and fourth toes have only three phalanges. Many birds, such as all Ratitae except _Apteryx_, have only three toes, the hallux being absent; in the Ostrich the second toe is also gone with the exception of a small metatarsal, so that the foot retains only the third and fourth digits, the third being much the larger of the two and bearing a claw, while the fourth is clawless. In the Swifts, Cormorants, and Penguins, all four toes are directed forwards. In most birds the hallux is directed backwards, and the other toes forwards. In the Owls the fourth toe can be directed backwards as well as the hallux, while in Parrots, Cuckoos, Woodpeckers, and Toucans the fourth toe is permanently reversed. In Trogons the second toe is reversed in addition to the hallux, but not the fourth. FOOTNOTES: [105] W.K. Parker, _Phil. Trans._ vol. 179, p. 385, 1888; and _Ibis_, 1888, p. 124. [106] See T.H. Huxley, "On the Classification of Birds," _P.Z.S._ 1867. [107] B. Lindsay, _P.Z.S._ 1885, p. 684. CHAPTER XX. CLASS MAMMALIA.