SKELETON. 



meeting its fellow in the median line below. The pelvic opening was 

 very small. The pelvic bones of the ichthyosaurs were weak, long and 

 slender, and apparently were imbedded in the muscles. 



In recent birds (figs. 50, 53) the pelvic bones are fused. The ilium 

 is greatly elongate and usually fused with the synsacrum (p. 53); ischium 



FIG. 119. FIG. 120. 



FIG. 1 19. Development of pelvis of chick, after Miss Johnson. A, chick of 6 days. 

 B, older; C, 20 days; cartilage dotted, bone white, a, acetabulum; il, ilium; is, ischium; 

 in, ischiadic nerves; on, obturator nerve; p, pubis; pp, pectineal process. 



FIG. 1 20. Pelvis of Galeopithecus, after Leche. ab, acetabular bone; i, ischium; il, 

 ilium; p, pubis; cartilage dotted. 



and pubis directed backward. The pubes, lying in the position of the 

 postpubes of the dinosaurs, never meet below except in the ostriches. 

 In the embryo (fig. 119) they are at first directed forward and only 

 attain the final position later. A pec- 

 tineal process arises from the aceta- 

 bular region and extends forward, simu- 

 lating the dinosaur pubis. 



In the mammals, obturator foramen 

 and ischio-pubic fenestra are united, 

 the opening being bounded on the 

 medial side by processes from ischium 

 and pubis. All three bones may meet in 

 the acetabulum, but more often the ex- 

 tension of ilium and ischium excludes 

 the pubis from the fossa. A peculiarity is the common occurrence of an 

 additional bone in the formation of the acetabulum (acetabular or coty- 

 loid bone). This lies between ilium and pubic bone and may fuse with 

 any of the elements. In marsupials and monotremes the interpubic car- 



FIG. 121. Left side of pelvis of 

 duck-bill, Ornithorhynchus, a, ace- 

 tabulum; il, ilium; is ischium; m, 

 marsupial bone; of, obturator fora- 

 men; p, os pubis; sv, sacral vertebra. 



