70 



VERTEBRATE SKELETON 



prepubic process. Later, the pubis gradually turns posteriorly, 

 finally paralleling the ischium, thus coming to a position similar 

 to that of the Ornithischian postpubis, the pubis of the latter resem- 

 bhng the prepubic process of many birds (fig. 295). 



In adult birds the bipedal posture demands a strong connexion 

 of pelvis and vertebral column, since the pelvis lies behind the centre 

 of gravity of the animal. Hence there is a great elongation of the 

 ilium, forwards and back, and its fixation to many more than 

 the primitive two sacrals, the result being the synsacrum (p. 48), 

 in which a varying number of vertebrae have united with the true 

 sacrum, and the ilia of the two sides have fused with these. The 

 union of pelvis and synsacrum is strengthened by the meeting of the 

 ilia of the two sides (sometimes for their whole length) dorsal 



Fig. 295. — Pelves of (.4) Polyborus (Shufeldt, '09) and (B) Picus pileatus. a, 

 acetabulum; il, ilium; ip, ischio-pubic fenestra; is, ischium; 0, obturator foramen; p, 

 pubis; V, vertebrae. 



to the vertebrae, and their fusion with the spinous processes and some- 

 times with the transverse processes of the synsacral vertebrae, 

 the spaces between iha and vertebrae often being more or less 

 occupied with bone. Usually the ilium is concave externally in 

 front and more convex behind, and it is most frequently pneumatic 

 of any of the pelvic bones. The acetabulum, always open, is near 

 the middle of the ventral margin of the ilium which sends down pre- 

 and postace tabular processes which fuse (except in ArchcBopteryx) 

 with pubis and ischium respectively, forming anterior and posterior 

 acetabular walls. 



Pubis and ischium are directed backwards and are relatively more 

 slender than in other groups, some Dinosaurs excepted. Both 

 are nearly parallel with the lower border of the ihum, the pubis 

 being usually the longer of the two. As a rule there is no symphysis 

 of either, the pelvis, except in Struthio, being widely open below. 

 Usually ischium and ilium are fused at both ends, enclosing an ilio- 

 schiadic fenestra. Primitively ischium and pubis are separate, but 



