Early Development of Appendicular Skeleton of the Ostrich. 365 



obturator foramen between, and in front a prepubis. The ostrich 

 chick seems to point back to something very similar. 



The most likely conclusion seems to be that somewhere in early 

 Triassic times a Ehynchocephaloid reptile, with thecodont dentition 

 and plate-like pelvis, gave rise to a small group of reptiles which 

 walked rather than crawled. This ancestral group had an expanded 

 ilium and well-developed limbs, and was probably in habits more 

 like a mammal than a reptile. From an arboreal member of this 

 group the pterosaurs have probably sprung. Others living more on 

 the ground had the hind legs better developed, and in course of time 

 came to be for the most part bipedal. Prom these bipedal forms 

 probably both birds and dinosaurs were derived. For a time the 

 birds balanced themselves like the dinosaurs by means of a heavy 

 tail, and while they continued to have heavy tails the pubis and 

 ischium would be directed mainly downwards. As the power of 

 flight gradually developed the tail probably gradually became lighter 

 and shorter, and to compensate for the loss of weight the pubis and 

 ischium were directed more and more backwards, to support the 

 muscles necessary to keep the body in an upright position. In the 

 carnivorous dinosaurs the heavy tail was retained, and much less- 

 muscular effort would be required to keep the body up, and hence 

 the ischium alone is developed in the posterior direction, and never 

 to the extent seen in birds. 



The relationships of the birds to the carnivorous dinosaurs and 

 pterosaurs may be represented thus : — 



Early Ehynchocephaloid ancestor 

 with plate-like pelvis. 



? ProterosucJms-like form. 



Sphenodon with 

 triradiating pelvis. 



Pterosauria. 



Theropoda 



Aves. 



The question might be discussed as to whether the common 



25 



