SKELETON 861 
acetabulum a thick process of the ilium descends to meet the os 
pubis, and behind a similar process meets the ischium. Behind 
the acetabular rim is a considerable thickening of the ilium, which 
frequently bears a facet on which plays the great trochanter of 
the femur. The postacetabular ilium is very variable in shape, 
either broadened out vertically only as in Columba and Rhea, or 
transversely, in the latter case forming a plain dorsal surface most 
Antitroch a! 
Fovea tliaca 
anterior 
PAVO CRISTATUS. 
pronounced in Pavo. The ischium originally extends backward, 
parallel to the postacetabular illum, and with it encloses the 
ischiadic notch. Among recent Birds this primitive condition 
persists only in Latite and Crypturi. In all others the distal por- 
COLYMBUS ARCTICUS. 
tions of the ilium and ischium meet so that this notch becomes a 
foramen, through which pass the big stems of the ischiadic nerves 
(page 625) and most of the chief blood-vessels of the hind limb.! 
The os pubis consists of an anterior and a posterior portion, the latter 
being long and slender, and running backwards more or less parallel 
1 A unique modification occurs in Rhea: the two ischia fuse in the middle 
line forming a long ischiadic symphysis which lies above the intestines separating 
them from the K1IpNEys. In adults the distal ends of the ilia fuse with these 
united ischia, forming foramina, and herewith is correlated a still more striking 
feature, namely the gradual resorbtion as maturity approaches of nearly the whole 
postsacral vertebral column, so that the caudal vertebre seem to be attached to 
the united ischiadic and iliac ossification. 
