iculations 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.

A slab from Turner's Falls, obtained for me by Dr. Deane in 1845,
measuring two feet by two and a half, and two inches in thickness,
contains at least ten different sets of impressions, varying from five
inches in length to two and a half, with a proportionate length of
stride from thirteen inches to six. All these are tridactylous, and
represent at least four diffe