THE FOSSIL FOOTMARKS OF THE UNITED STATES. 241 



ill Gill, where the highly inclined shales are laid bare. Mr. Marsh 

 has generously allowed me to take sketches from his specimens, 

 and to give the species scientific names ; although he expects to 

 give a popular description of them, in the American Journal of Sci- 

 ence, before the publication of this paper. 



Among Mr. Marsh's specimens, found at the above-named local- 

 ity, is one of which a sketch of two rows of tracks, reduced three 

 times, is given on Plate 23, figs. 1 and 2. I cannot satisfactorily 

 refer this track to any known species, though perhaps it may be- 

 long to the one last described ; that is, an impression considerably 

 below the layer on which the animal trod. It is chiefly remarkable 

 for the axis of the foot being turned so much inward, towards the 

 line of direction, and for the wire-like fineness of the extremities 

 of the toes. But the different tracks are so unlike and so imper- 

 fect, that I conclude they are a good deal altered from the original, 

 and prefer not to describe them as a new species. 



Affinities of the Group. — One cannot look at the succession of 

 tracks and the form of the feet in this group, as exhibited upon 

 the accompanying drawings, and much less upon the originals, 

 without being struck with their resemblance to the feet and the 

 tracks of small Lacertilia. The number of toes, indeed, corresponds 

 perhaps more nearly to certain batrachians, say the Salaman- 

 dridcB and Sirenidce, which very commonly have only four toes, at 

 least on the fore feet. But the long heel corresponds better to the 

 lizards ; and, upon the whole, I incline to consider them as such. 

 And yet it is extremely difficult to decide between these two 

 classes. There is one fact, especially, in respect to the first two 

 species of Macropterna, that does not well correspond to either 

 tribe. 1 mean the small deviation of the animal's feet to the right 

 and the left of the line of direction. What living Lacertilia or 



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