15 
in the Paris gypsum by Cuvier previous to 1830. 
Since that time, they have been found in the Lower 
Eocene in England, and the Swiss Alps; and there is 
reason to believe that osseous relics may be met with 
in the same deposits which contain the foot-marks. 
Most of the bird-tracks which have been observed, 
belong to the wading birds, or Gralle. 
The number of toes in existing birds varies from 
two to five. In the fossil bird-tracks, the most 
frequent. number is three, called tridactylous; but 
there are instances also of four or tetradactylous, and 
two or didactylous. The number of articulations 
corresponds in ornithichnites with living birds: when 
there are four toes, the inner or hind toe has two arti- 
culations, the second toe three, the third toe four, the 
outer toe five. The impressions of the articulations 
are sometimes very distinct, and even that of the skin 
covering them. 
President Hitchcock has distinguished more than 
thirty species of birds, four of lizards, three of tor- 
toises, and six of batrachians. 
The great difference in the characters of many fossil 
animals from those of existing genera and species, in 
the opinion of Prof. Agassiz, makes it probable that 
in various instances the traces of supposed birds may 
be in fact traces of other animals, as, for example, 
