Page:The Osteology of the Reptiles.pdf/168

150 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 vestigial (Ankylosaurus), but is broad and stout in the quadrupedal Ceratopsia, where apparently it again functions as the normal pubis. The postpubis, 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 quadrupedal Ceratopsia, which have certainly descended from bipedal forms. It is, however, unusually stout in the quadrupedal Stegosaurus, 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 separate 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 postpubic 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