6. The Pelvic Arch consists of the Ilium, Ischium, and Os pubis, these three paired bones meeting from each side at the cup (_acetabulum_) that receives the head of the femur, and coalescing early in life; while the _incisura ischiadica_ or notch between the ischium and the ilium becomes an inclosed space (_foramen_) in all Birds except the _Ratitae_ and _Crypturi_. [Illustration: FIG. 4.–Pelvis of _Apteryx australis_. Lateral view. _a_, Acetabulum; _il_, ilium; _is_, ischium; _p_, pectineal process of pubis; _p^1_, pubis. (From Wiedersheim, after Marsh.)] 7. The Posterior Limbs, or Legs, are composed of the Femur or thigh, the Tibia and Fibula, making the shank or "drumstick," and the bones of the Foot. The thigh, however, being hidden by the plumage, the shank of a Bird might easily be taken for the thigh, and the metatarsus (the cannon-bone of some) for the shank. The tibia and fibula commonly unite to some extent, and the former, as it now exists in adult Birds, is strictly a "tibio-tarsus," since with it is fused the proximal portion of the originally existing tarsal elements. Similarly the distal tarsal {10}elements unite with the metatarsus, which is therefore properly a "tarso-metatarsus," though often called merely "tarsus" by ornithologists. This arises from a fusion of the second, third, and fourth metatarsal bones, which in the adult (except among the _Sphenisci_ and to some extent in _Psittaci_) do not lie in the same plane; the middle one having its upper end thrust backward and its lower end forward in the course of growth to maturity. The fifth metatarsal practically disappears, while the first remains more or less separate, and lies behind the distal portion of the other metatarsals.