160 AN ATTEMPT TO DESCRIBE THE ANIMALS THAT MADE 



legged, and some long-legged. Some may suppose that these dif- 

 ferences only show that the animals moved with different rapidity 

 at different times. There is, indeed, a considerable diversity in the 

 length of the step of the same species on different specimens ; but 

 such cases as the Brontozoum parallelum, Typopus abnormis, Ani- 

 sopus Deweyanus, and gracilis, at one extreme, and Otozoum 

 Moodii at the other, make it evident that each animal had its pecu- 

 liar type of progress and of stride. Yet there is so much difference 

 in that stride, at different times, that I have not depended on that 

 character alone to establish a species. 



In giving the length of the step in the quadrupedal tracks, I have 

 measured from track to track of the same foot. 



20. Size of the foot. In a few instances the species of foot- 

 marks scarcely differ except in size ; the best example of which is 

 in the genus Steropezoum, whose three species (Plate 5, figs. 1-3) 

 resemble one another in form, although I have seldom seen the pe- 

 culiar heel of the ingens and elegans upon the elegantius, and the 

 first two differ considerably in the ratio between the length of the 

 middle toe and its extension beyond the two others. The question 

 arises, whether the smaller species should not be considered as the 

 young of the other. This is possible. But then we ought to find 

 specimens of every intermediate size, which has not yet been done. 

 And besides, is it probable that very young animals would often fre- 

 quent such thoroughfares as the localities of footmarks seem to 

 have been, where so many sorts of animals resorted, and where, 

 in the dearth of food that must sometimes have existed, the young 

 ones must often have been devoured if present ? Are living ani- 

 mals wont to bring their offspring into such places, till they have 

 attained considerable size ? 



Considerations like these have led me to the conclusion, that 



