THE FOSSIL FOOTMARKS OF THE UNITED STATES. 147 



tinct ; and hence I doubt not that the upper track is smaller chiefly 

 from the filling in of materials upon the original impression. 



These examples, to which I might add more, show how careful 

 we ought to be not to confound the impressions of the same track 

 on different layers with different species. Nothing but long expe- 

 rience in ichnolithological researches will prevent such mistakes. 



The number of toes (to return to the character which we were 

 considering) varies from three to five ; though, if the sketches on 

 Plate 18 are the tracks of animals, we might call them didactylous. 

 But they are so anomalous that I leave them out of the account, 

 especially as they may belong to invertebrate tribes, if they are 

 indeed real tracks. 



From the details that have been given, we see that this char- 

 acter (the number of toes), although important, is in some cases 

 of difficult determination. 



4. Absolute and relative length of the toes. — In these characters 

 there is a good deal of constancy ; and hence they afford good 

 grounds for specific and even generic distinctions. There are, how- 

 ever, some difficulties in the determination of these points. One is, 

 the uncertainty that often exists, whether the track before us exhib- 

 its the very surface on which the animal trod. If it be above or 

 below that plane, the toes will always be too short, although their 

 relative length (the most important character) may not be essential- 

 ly altered. But the greatest difficulty lies in determining how far 

 backward the toes extend ; that is, where the toes end and the 

 heel begins. In the thick-toed tracks, this point can generally be 

 decided with accuracy ; though it hardly can be in the case of the 

 anomalous Otozoum. But in the narrow-toed tracks, especially if 

 they are digitigrade, and if their divarication is small, we can get 

 only an approximate measurement of the length of the toes. The 



