THE FOSSIL FOOTMARKS OF THE UNITED STATES. 247 



Group X. 



Feet didactylous ; toes unequal, in shape somewhat like the 

 drag used in tilling land. 



Genus XXIII. HARPAGOPUS. 



Characters the same as those of the group. 



Remarks, — I have hesitated long before referring the marks 

 described under this group to the tracks of animals, because they 

 differ so much from the feet of any animals with which I am ac- 

 quainted. But there is so much uniformity among these impres- 

 sions, that we must refer them to some common cause; some cause, 

 too, that made an impression on the surface of mud, rather than to 

 a body interposed between layers of mud ; and I know of no agen- 

 cy, but the feet of animals, that could have made such impressions. 

 Moreover, we do know of some living animals (as the crustaceans), 

 that have didactylous feet. Heteroclitic, then, as these mark- 

 ings are, I must refer them to the tracks of animals, till proved to 

 be something else. 



Species 1. Harpagopus giganteus. (PI. XVIII. Fig. 1.) 



Nos. 137, 152, in Cabinet. 



Divarication of one pair of toes, 15°; of the other, 25°. Length 

 of the longest toe in one pair, 10.5 inches ; of the shortest do., 7 

 inches ; of the longest in the other pair, 1 .3 inches ; of the short- 

 est do., 5 inches (as far as it reached the ground). Thickness of 

 the toes, 1.4 to 1.7 inch. Feet pointing in nearly opposite direc- 

 tions. One foot shown, of the natural size, on Plate 18, fig. 1. 

 On Plate 23, fig. 5, is a reduced copy of the slab, showing both 

 feet, and also a row of the tracks of Brontozoum parallelum and 

 JEthyopus minor. 



