THE POSE AND LOCOMOTION OF DIPLODOCUS I9 



resembling in this respect the feet of the crocodiles. A reason for 

 this conclusion is found in the fact that all the feet of the sauropods 

 are entaxonic and also the fore feet of the earliest known theropods. 

 It is therefore more probable that the hinder feet of the latter 

 reptiles became mesaxonic from an entaxonic condition than that 

 their fore feet and both fore and hind feet of the sauropods should 

 be transformed. That the manus of the theropods was entaxonic 

 may be seen from Marsh's figure of the fore foot of Anchisaurus 

 colurus and A. polyzelus (Dinosaurs N. A., pis. ii, iii) and from Dr. v. 

 Huene's figures. Furthermore, the hinder feet of the early theropods 

 present plain indications of a former entaxonic arrangement. The 

 foot of Ammosaurus^^ shows a very stout first digit, not greatly shorter 

 than the others, while the second does not fall behind the third and 

 fourth in diameter of the bones, little in length. The superiority of 

 the second to the third seems to have been retained in Allosaurus. 

 When the hind leg began to be drawn forward against the side and 

 the weight of the body was throwTi to a greater extent on the median 

 digits a stimulus appears to have been given to the development of 

 the third digit, while the first, relieved to some extent of its former 

 duty, became reduced and turned backward. 



In the later theropods the manus also became mesaxonic. This is 

 seen in Marsh's restoration of the skeleton of Ceratosaurus (op. cit., 

 pi. xiv). Mr. C. W. Gilmore, who has recently mounted this skeleton 

 has shown me the remains of the one hand preserved. Most of the 

 phalanges are missing. There are present four metacarpals, and 

 there are no traces of the fifth in the rock. The first is considerably 

 reduced, the second is the largest. Thus, there is evidence that all 

 the feet of the carnivorous dinosaurs became transformed from the 

 entaxonic to the mesaxonic condition. It further appears that the 

 sauropods retained the primitive condition of the feet, fore and hinder, 

 more persistently than did the other groups of the order. 



For reptiles that progress by creeping, having the humerus and the 

 femur at right angles with the body in the middle of the step, the 

 entaxonic condition seems most effective. It is found in the croco- 

 diles and the turtles, being especially well displayed in the triony- 

 chids and the land tortoises. In reptiles the first digit is usually 



" Marsh, op. cit. pi. iii, fig. 6. 



