| 1. ilium. | 5. acetabulum. |
| 2. ischium. | 6. ilio-sciatic foramen. |
| 3. pubis. | 7. fused vertebrae. |
| 4. pectineal process. | 8. antitrochanter. |
The ischium (figs. 58 and 61, 2) is a flattened bone which forms about one-third of the acetabulum, and lies ventral to the posterior part of the ilium. Its anterior portion is separated from the ilium by the large oval ilio-sciatic foramen (fig. 61, 6), while behind this the two bones are completely fused.
The pubis (figs. 58 and 61, 3) is a very long slender bar of bone which forms only a very small part of the acetabulum and runs back parallel to the ventral surface of the ischium with which it is loosely connected at its posterior end. For the greater part of their length the two bones are separated by the long narrow obturator foramen. Behind the ischium the pubis is produced into a long curved downwardly-projecting process, and in front of the acetabulum it bears a short blunt pectineal or pre-pubic process (fig. 61, 4) probably homologous with the pre-pubis of Orthopod Dinosaurs. The remainder of the pubis is homologous with the post-pubis of Orthopod Dinosaurs.
The Posterior Limb.
The leg of the bird is somewhat differently constructed from that of other vertebrates owing to the fact that there is no free tarsus, the proximal tarsals having fused with the tibia, and the distal with the metatarsals.
The thigh consists of a single bone, the femur. The femur is a comparatively short bone with a straight shaft and expanded ends. The proximal end bears on its inner side a rounded head, which articulates with the acetabulum. On its outer side is an irregular outgrowth, the great trochanter, while between the two is the surface which meets the antitrochanter of the ilium. The posterior end also is expanded and marked by a wide groove which lodges the patella. On each side of the groove is a strong condylar ridge for articulation with the tibia. The external condyle is deeply grooved behind for articulation with the fibula.
The crus or shin consists of two separate bones, (1) the tibio-tarsus, formed by the fusion of the tibia with the proximal row of tarsals, and (2) the fibula.
The tibio-tarsus is a thick straight bone nearly twice as long as the femur. Both ends of the bone are considerably expanded. The proximal end bears two slight depressions which articulate with the condyles of the femur, and a third depression which partly lodges the patella. The proximal end of the anterior or extensor surface is drawn out into a very prominent cnemial crest which bends over towards the postaxial side of the bone; a slight ridge is continued from it all the way down the shaft. The proximal part of the shaft of the tibio-tarsus bears a roughened ridge with which the fibula is closely connected. The distal end is expanded and rotated outwardly and forms a prominent pulley-like surface which articulates with the tarso-metatarsus.
The fibula is reduced to the proximal portion only, which is expanded and articulates with a depression behind the external condyle of the femur. The fibula further extends about a third of the way down the shaft of the tibio-tarsus. The patella or knee-cap is a sesamoid bone due to an ossification in the tendon of the extensor muscles of the leg.
The ankle joint lies between the proximal and distal tarsals which as previously mentioned fuse respectively with the tibia and metatarsus.