THE FOSSIL FOOTMARKS OF THE UNITED STATES. 143 



that, too, only upon the uppermost layer. While the otiier toes 

 seem to have depressed the layers of mud an inch or two, or more, 

 in depth, this one reaches only a slight distance downward. 

 Hence we often obtain specimens, apparently very perfect, in 

 which the hind toe is wanting, when in fact it was present on a 

 higher layer. The same liability to deception occurs in some cases 

 when a short toe was attached to some part of a long heel, as it is 

 in some reptiles. It might be only very rarely that it made an im- 

 pression, save perhaps upon the highest layer. 



The changes that take place in tracks in a vertical direction, that 

 is, on successive layers of rock, is one of the most fruitful sources 

 of error as to their true character and the number of toes. I have 

 specimens which show the same track, or parts of it, to the depth 

 of four or five inches ; and if such a rock be split in different places, 

 it will often show considerable diversity of forms, and yet it may 

 be that all of them shall be quite distinct ; so that, if we have only 

 one layer, it is very difficult often to determine whether it was the 

 identical layer on which the animal trod, or one above or below it. 

 In following a track downward, the hind toe, if it had one, usually 

 first disappears; next the heel, then the lateral toes, while the cen- 

 tral one sinks the deepest. 



In the plates annexed, I have given several examples of the 

 changes that occur in tracks in a vertical direction, as they are 

 shown upon successive layers of the rock. These, however, I ought 

 to remark, are rather extreme cases. Plate 15, figs. 10- 13, exhibits 

 a track of Tricenopus Baileyanus on four successive layers, the whole 

 about two inches in thickness, fig. 10 being the uppermost layer. 

 The dotted lines around the heel will be described in a subsequent 

 part of this paper. Figs. 14-16 of the same plate show the 

 Tricenopus Emmonsiamis on successive layers, but little more than 



