4 OLIVER P. HAY 



Kent's figures 3 the hinder limbs are not carried backward and forward 

 in sagittal planes like those of mammals. 



It seems not difficult to understand the history of the attainment 

 of the bipedal habit among lizards and dinosaurs. When the fore- 

 legs of a quadrupedal reptile are of nearly the same length and have 

 the same structure as the hind legs there seems to be no good reason 

 why the animal cannot run as fast on four legs as on two. However, 

 the hinder limbs, being nearer the center of gravity of the animal, 

 receiving more of the weight, and being more devoted to propulsion 

 of the body, are likely to become larger and more powerful, while the 

 fore legs may become more or less reduced, with or without special 

 modification for other purposes. If now a reptile whose fore legs 

 have become relatively much shorter than the hinder ones has occas- 

 sion to run with the greatest possible speed, it is likely to find that the 

 fore legs cannot take as long steps as the hinder ones; and naturally 

 it endeavors to get them out of the way by lifting them up in the air. 



This practice would be of great advantage and would tend to become 

 fixed. The reduced limbs might then become modified for other 

 purposes or undergo further reduction. In the beginning, the femora 

 would stand out from the body, giving the animal a wide tread. In 

 time, however, the knees might be drawn closer to the flanks, the 

 tread would become narrower and the pace more rapid. At no stage, 

 however, would the reptile walk like a quadrupedal mammal; and no 

 argument in favor of such a gait or Diplodocus can b 3 deduced from 

 bipedalism in lizards. 



If the mammal-like gait of Diplodocus be insisted upon on the 

 ground of straightness of the femur it may be pointed out, as I did 

 in the article in the American Naturalist, that the femora of sphenodon 

 and of lizards, animals that creep, are straight. If it be contended 

 that it is in the heavy-bodied animals that a straight femur is corre- 

 lated with a lifting of the body from he ground during locomotion, 

 it may be permitted to recall that the femora of Allosaurus and Tyran- 

 'nosaurus, great carnivorous dinosaurs, are distinctly bent. The 

 femora of Trachodon are straight, while those of Camptosaurus and 

 IsLOsaurus are curved. Curvature of the femur seems, therefore, to 

 have no relation to size of body or erectness of pose. The femora of 



8 Nature, vol. 53, 1895, pp. 396-397. 



