6 OLIVER P. HAY 



conviction that the original surfaces, barring a thin layer of cartilage, 

 are preserved; there is a definite head, separated from the shaft by a 

 distinct neck and nearly filling the acetabulum; and there is a definitely 

 formed trochanter major. In the Sauropoda, on the contrary, the 

 shaft seems to be composed of coarser bone; the articular surfaces are 

 rough and show that they were covered by a thick layer of cartilage; 

 the head merges imperceptibly into the supposed great trochanter 

 and into the shaft; and the head lacks much of filling the acetabulum. 

 In its low stage of differentiation the femora of the sauropods resemble 

 greatly those of the crocodiles and are hardly above those of the 

 lizards. They furnish no warrant for the belief that their possessors 

 walked in mammalian fashion. 



The structure of the foot of Diplodocus indicates that this reptile 

 walked in a way very different from that in which the bipedal dinosaurs 

 walked. In the latter the foot had the third toe most strongly de- 

 veloped (mesaxonic) ; in the sauropods the two inner toes were the 

 strongest, the third somewhat weaker, while the other two were 

 greatly reduced. This difference of structure must have had its 

 history and its meaning. That the feet of Diplodocus were shortened 

 and more or less digitigrade indicates that they were employed for 

 walking, not at all for swimming. The feet of the crocodiles are to be 

 regarded as entaxonic, the inner digits being of stouter build, although 

 slightly shorter than the third; but here the digits are elongated and 

 webbed to assist in swimming. When the animal is walking, the 

 pressure comes against principally the inner side of the foot. The 

 trionychid turtles have the three inner digits most strongly developed 

 and clawed; the others are slender and unarmed. The clawed digits 

 are, of course, the ones employed for excavating hiding places in the 

 sand and mud and getting foothold in walking and running; and 

 these turtles are, for moderate distances, rapid and powerful runners 

 on the land and on the bottoms of streams. 



It is true that the foot of man is entaxonic and is directed nearly 

 forward, but its history is wholly different from that of the sauropod 

 foot. It is certain that the ancestors of man were climbing animals, 

 with hallux strongly developed and opposable to the other digits. 

 Being later employed for locomotion on the ground, the foot under- 

 went a transformation to its present form. The form assumed at any 

 time by an organ must depend greatly on the form previously pos- 



