THE POSE AND LOCOMOTION OF DIPLODOCUS 17 



the femur has been quoted above. To the writer it seems probable 

 that the whole proximal end of the bone constituted the head and was 

 inserted into the acetabulum, as in lizards and crocodiles, and that 

 the thigh was directed outward still more than Dr. v. Huene has 

 supposed. 



What then made those bird-like tracks that are so abundant in the 

 sandstones of the Connecticut River valley ? Why not birds, indeed ? 

 Although remains of birds have not yet been found in Triassic rocks 

 there can be little doubt that these animals had already freed them- 

 selves from the dinosaurs. Already long before the close of the Juras- 

 sic the hinder limbs of birds had, as we learn from Archceopteryx 

 taken on its present form, with doubtless ability to plant its footsteps 

 on the line of direction. This limb was at that early time far in 

 advance of the hind leg of the dinosaurs of even the Upper Cretaceous; 

 and it was doubtless even in the Triassic far in advance of the limb of 

 the dinosaurs of that time. No bird remains have been found where 

 those famous tracks occur, it is true. It is also true that nearly 100 

 kinds of tracks have been distinguished, while only 8 or 10 species of 

 dinosaurs have been discovered in the North American Triassic; and 

 of these only one has had its tracks identified. Therefore, it seems 

 to the writer entirely reasonable to suppose that those bird-like tracks, 

 even some of them that show the presence of fore feet and tail, were 

 really made by birds. For if the birds diverged from the dinosaurs 

 early in the Triassic their wings were as yet probably unfitted for con- 

 tinuous flight in the air. Many of them were probably running ani- 

 mals and some of them may still have retained a tendency to grow 

 to a large size. Success in flying necessitated in later times a reduc- 

 tion in size of body. In the Trias the hands had not yet become 

 reduced and transformed through the development of great pinion 

 feathers, and they may have been at times applied to the ground in 

 walking and resting. The tail was yet long, little befeathered, and 

 might drag on the ground and leave a trail. And it must not be 

 regarded as wholly certain that the tracks of large bipedal animals 

 of later times were made by dinosaurs. There may have been in the 

 Jurassic and the Cretaceous, as well as in the Tertiary, running birds 

 of even greater size than the largest moa, whose foot was hardly 

 inferior in size to that of many dinosaurs. On the other hand, such 

 dinosaurs as Compsognaihus and Hallopus may have walked like 

 Proc. Wash. Acad. Sci. February, 1910. 



