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 articulations, 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 tortoises, 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, 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 Elementaire de Paleontologie et de Geologie," 5 vols. 1849-52), does not accept the arrangement of President Hitchcock. He objects to the term Ornithichnites, and proposes what he considers a more comprehensive arrangement into organic, physiological, and physical impressions. _Organic impressions_ are those which have been produced by the remains of organized 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 continent in the carboniferous strata of Nova Scotia, and of the Alleghenies; 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 Massachusetts to Middletown in Connecticut, a distance of about eighty miles.