24 OLIVER P. HAY 



betook themselves to the streams, in order to enjoy the advantages of 

 easier transportation; and then they became still more massive. Had 

 they originally been aquatic and had they continued so, their feet 

 would have remained more like those of crocodiles, less digitigrade 

 and less shortened than they were in Diplodocus. 



In his paper on the relationships of the birds and the dinosaurs 17 

 Professor Osborn says: 



Thus tridactylism is correlated with rapid bipedal progression, the inner 

 and outer digits suffering reduction. 



In formulating this apparently important generalization Professor 

 Osborn did not qualify it with the statement that most of the so-called 

 tridactyl animals are really tetradactyl, the hallux being present and 

 usually functional. Nor could he have had before him the skeleton 

 of any of the sloths, animals that are strictly tridactyl behind, but 

 which are neither bipedal nor endowed with great speed. Tridactyl- 

 ism prevailed among the extinct horse-like perissodactyls and is a 

 characteristic of modern tapirs. On the other hand, there may exist 

 swift bipedal progression independently of tridactylism. The ostrich 

 makes rapid headway with only two toes, one might almost say, with 

 a toe and a half. The kangaroos are wonderful bipedal leapers, 

 whose functional digits are reduced to two, the fourth and the fifth. 

 Man may be justly counted among the swift runners, trained individ- 

 uas making their mile in four and a quarter minutes, and he possesses 

 a pentadactyl entaxonic foot. No bipedal artiodactyl is recalled, but, 

 as illustrating a possibility, one must not forget to mention Pan, the 

 shepherd god of old Arcady. From which considerations it may be 

 concluded that the bipedal rapid runners have adopted no standard 

 form of foot. 



Accompanying the present paper is a drawing (PL I) which is 

 intended to represent the habits of Diplodocus, especially as regards 

 its habitual pose of body and its manner of locomotion, as conceived 

 by the writer. This drawing was executed by Miss Mary Mason 

 Mitchell, after consultation with the author of the paper. Two indi- 

 viduals are in the foreground. One is collecting food from the sur- 

 face of the water; the other has the head high in air and is jealously 



17 Amer. Naturalist, xxxiv, 1900, p. 796. 



