150 THE OSTEOLOGY OF THE REPTH^ES 



pubic process, typically flattened and more or less spatulate distally, 

 is directed forward and downward [upward] and does not join its 

 mate in a median symphysis. At times it may be small or even ves- 

 tigial (Ankylosaurus) , but is broad and stout in the quadrupedal 

 Ceratopsia, where apparently it again functions as the normal pubis. 

 The postpuhis, or postpubic process, typically is long and slender, 

 directed backward immediately below, the slender ischium and not 

 meeting its mate in a symphysis; that is, the pelvis is more or less 

 open below, as in birds. The postpubis is vestigial in the heavy quad- 

 rupedal Ceratopsia, which have certainly descended from bipedal 

 forms. It is, however, unusually stout in the quadrupedal Stego- 

 saurus, possibly as a reinforcement to the ischia in the support of the 

 heavily armored body. 



When this peculiarity of the dinosaurian pelvis was first discovered 

 by Hulke and Marsh it was hailed as a direct proof of the dinosaurian 

 ancestry of birds. It may be, however, merely another of the many 

 parallel characters brought about by similar causes. According to 

 one view, the prepubic process is the real pubis, homologous with the 

 pubis of the Saurischia; the postpubic process an outgrowth from it. 

 According to another view, the postpubic process is the real pubis, 

 corresponding to the pubis of birds, the prepubic process homologous 

 with the prepubis of pterodactyls or crocodiles. There has never 

 been, however, any evidence to show that it is derived from a sepa- 

 rate center of ossification. 



An analogous but not homologous structure is observed in many 

 running birds, the ostriches, Geococcyx, etc., where, in addition to 

 the normal, slender, posteriorly directed pubis similar to the post- 

 pubic process of the dinosaurs, a more or less prominent pectineal 

 process, arising, however, from the ilium, is directed forward, like that 

 of the dinosaurs. The pubis of birds in its embryonic development 

 turns backward from its normal position. Whence it would appear 

 that the development of the two processes in the dinosaurs has 

 arisen in response to similar causes, and cannot be ascribed to a 

 common heredity, as was once thought. Why the bipedal predentate 

 dinosaurs should have acquired such a remarkable structure of the 

 pelvis, and not the even more bipedal theropods, is not yet entirely 

 clear. It has been ascribed to differences in the posture of the tail in 

 running, but would seem, to the author at least, rather to have been 



