THE SKELETON IN BIRDS. THE PELVIC GIRDLE. 341 



birds it fuses posteriorly, further forward the ilio-sciatic 

 foramen separates the two bones. In Tinamus, Hesperornis, 

 Apteryx (fig. 65, B, 2), and Struthio, the ischia are separate 

 from the ilia along their whole length except at the acetabu- 

 lum ; in Phororhacos, on the other hand, the two bones are 

 fused along almost their whole length. The bone usually 

 called the pubis in birds corresponds to the post-pubis of 

 Dinosaurs and forms a long slender rod (fig. 65, 3) lying 

 parallel to the ischiurn. In many birds the ischia and pubes 

 are united at their distal ends. This is the case in the Ostrich 

 (fig. 65, D), in which the ilia and ischia are widely separated. 

 In many birds the pubis is drawn out in front into the pectineal 

 process, this is specially large in Apteryx (fig. 65, B, 5), and in 

 the embryos of many birds. It is probably homologous with the 

 pre-pubis of Dinosaurs but in some birds is formed in part by 

 the ilium. The acetabulum in birds is always perforate. 



In Rhea (fig. 65, C, 2) and probably in Archaeopteryx a 

 symphysis ischii occurs, and in the ostrich alone among birds 

 there is a symphysis pubis. In Archaeopteryx all three bones 

 of the pelvis are distinct, but they are imperfectly known. In 

 Ichthyornis they are also distinct, in all other known birds 

 they are fused together to a greater or less extent. 



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 sEpyornis. 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 com- 

 plete. The distal tarsals fuse with the second, third and fourth 



