The ilium in reptiles usually has a more or less prolonged process or projection turned backward by the side of the anterior caudal vertebrae, but in those animals which walked erect on the hind legs, the dinosaurs and pterodactyls, as also some of the more erect-walking reptiles ancestral to the mammals, this process was directed forward, as in birds and mammals. The crocodilia, unlike all other known reptiles, have the pubes excluded from the acetabulum, and they do not meet in a median symphysis. This character alone will distinguish any crocodilian from all other reptiles. But there is some doubt as to the homology of the bones usually called pubes in the crocodiles. Some of the bipedal dinosaurs have the pubis forked, the anterior part directed downward and forward, and not meeting its mate in a symphysis, the posterior process long and slender, lying below the long ischium, as in birds. Indeed, when this peculiarity of the dinosaurian pubis was first discovered, it was thought to be an evidence of the immediate relationship of birds; its structure is now interpreted differently. POSTERIOR EXTREMITY The thigh bone or femur in reptiles, like the humerus, is variable in size and shape. Only in those reptiles that walked erect is the articulation of the head set off from the shaft of the bone by a distinct neck. In others the articulation is at the extreme top of the bone, since the thigh bones are habitually turned more or less directly outward from the acetabulum and the long axis of the body. The more or less pronounced rugosities at the upper end of the femur, for the attachment of muscles, called trochanters, are not easily distinguishable into the greater and lesser, as in mammals. Sometimes, as in the erect-walking dinosaurs, there is a more or less pronounced process on the shaft lower down, called the fourth trochanter, for the attachment of caudal muscles. On the back part of the shaft there is a ridge or line for the attachment of muscles, corresponding to the linea aspera of the mammalian femur. The projections at the lower end on the sides are called condyles. The two bones of the leg, or shin, are usually shorter than the thigh bones, though in running and leaping animals they may be quite as long or even longer. That on the inner or big toe side is called the tibia, and articulates with the distal end of the femur, but chiefly with its inner condyle. It has a more or less well-developed crest in front above for the attachment of the extensor muscles directly, since there never is a patella in reptiles, and only rarely sesamoid bones of any kind. The fibula, at the little-toe side of the leg, is usually more slender than the tibia, though it may be larger in swimming reptiles and even in some running forms. It disappeared in some of the later pterodactyls. Its upper articulation has a more gliding and somewhat rotary motion on the outer condyle of the femur, turning the foot outward in extension of the leg.