16 
those of the lizard or frog. And he supports this 
opinion, among other reasons, by the disappearance of 
the heel in a great number of Ornithichnites. 
D’Orbigny, to whom we are indebted for the most 
ample and systematic work on Paleontology (“Cours 
Elémentaire de Paléontologie et de Géologie,” 5 vols. 
1849-52), does not accept the arrangement of Presi- 
dent Hitchcock. He objects to the term Ornithich- 
nites, and proposes what he considers a more compre- 
hensive arrangement into organic, physiological, and 
physical impressions. Organic impressions are those 
which have been produced by the remains of organ- 
ized substances, such as vegetable impressions from 
calamites, &c. Physiological impressions are those 
produced by the feet and other parts of animals. 
Physical impressions are those from rain-drops and 
ripple-marks; and to these may be added coprolites in 
substance. This plan of D’Orbigny seems to exclude 
the curious and interesting distinctions of groups, 
genera, and species; in this way diminishing the 
importance of the science of Ichnology. 
Fossil impressions have been found on this conti- 
nent in the carboniferous strata of Nova Scotia, and 
of the Alleghanies; in the sandstone of New Jersey, 
and in that of the Connecticut Valley in a great 
number of places, from the town of Gill in Massa- 
