THE FOSSIL FOOTMARKS OF THE UNITED STATES. 133 



open several rods in thickness, and sometimes, as at Turner's Falls, 

 more than a quarter of a mile ; and thus we can easily learn what 

 animals trod upon the deposits through a series of thousands of 

 years : for we can hardly suppose, that, in such fine sediment as 

 that which composes these rocks, the accumulations could have 

 been more than an inch or two each year. 



Consider, thirdly, that we usu;.lly find the tracks limited to a 

 belt of rock only a few feet wide, which formed the shore of the 

 ancient estuary. Along this pathway, we should naturally expect 

 to find the tracks of all the animals that trod those ancient shores. 



Suppose, now, that only as many animals of this kind formerly 

 lived in this valley as now do, — and since the climate was then 

 tropical, and that was the period when the batrachian, lacertilian, 

 and chelonian races were greatly developed (to say nothing of 

 Struthionidae), this cannot be regarded as an extravagant supposi- 

 tion, — might we not expect to find, at so many localities, and on 

 so many hundred successive layers of rock, as many as forty-seven 

 species of animals capable of being distinguished by their tracks ? 

 for we do not suppose that all species can be thus distinguished. 

 However, it would be strange if I should not have sometimes been 

 mistaken as to species, where they must be described only from 

 their tracks, and, in consequence of imperfect specimens, have 

 made two species out of one. After I have described the whole, 

 naturalists can better judge on this point ; and my only wish is to 

 have all species dropped that have not good distinctive characters. 

 The species which I regard as the most uncertain are the Bron- 

 tozoum expansum, Steropezoum elegantius, Argozoum Redfieldi- 

 anum and mininum, Platypterna Deaniana, Ornithopus Adamso- 

 nus, Plectropus minitans, Triaenopus Emmonsianus, Anisopus gra- 

 cilis, and the three species of Harpagopus. If all these should turn 



