- Toes thick, pachydactylous; or thin, leptodactylous.
- Feet winged.
- Number of toes from two to five, inclusive.
- Absolute and relative length of the toes.
- Divarication of the lateral toes.
- Angle made by the inner and middle, outer and middle toes.
- Projection of the middle beyond the lateral toes.
- Distance between tips of lateral toes.
- Distance between tips of middle and inner and outer toes.
- Position and direction of hind toe.
- Character of claw.
- Width of toes.
- Number and length of phalangeal expansions.
- Character of the heel.
- Irregularities of under side of foot.
- Versed sine of curvature of toes.
- Angle of axis of foot with line of direction.
- Distance of posterior part of the foot from line of direction.
- Length of step.
- Size of foot.
- Character of the integuments of the foot.
- Coprolites.
- Means of distinguishing bipedal from quadrupedal tracks.
By these characters, President Hitchcock has distinguished physiological tracks, or those made by animated beings, into ten groups provisionally. To these may be added, "organic impressions," made by organized bodies; and the impressions made by inanimate bodies, called "physical impressions."
The specimens under our hands enable us to give some notion of the distinctions which characterize the greater part of these groups.
GROUP FIRST—STRUTHIONES.
The ostrich-tracks present a numerous natural and most remarkable group; remarkable from the great size of some species,—all of them tridactylous and pachydactylous. The ostrich of the Old World has only two toes, but this family exists in South America at the present time under the name of Rhea Americana; and tracks of an animal, probably of the same family, are found in the numerous impressions near Connecticut River,—all of them having three toes in front, and the rudiment of a fourth behind.
This group contains a number of genera. The First Genus, denominated Brontozoum, presents the tracks of a most extraordinary bird. These tracks appear less questionable since the discovery in Madagascar of the eggs of the Epyornis.
The tracks of the largest species, the Brontozoum Giganteum, are four times the magnitude of those made by the existing ostrich of Africa. They are very numerous, and congregated together. The foot of the Brontozoum Giganteum, including the inferior extremity of the tarso-metatarsal bone, which makes a part of the foot, measures in our specimen twenty inches; in the [Mastodon Giganteus,] the foot measures twenty-seven inches; the width also is less, being ten inches across the metacarpals, while that of the Mastodon is twenty-two: but the one is a bird, the other a quadruped. The toes are three in number, and present the same divisions with existing birds; the inner toe having three, the middle four, the outer five phalanges. Some of the articulations of the toes of this noble specimen are remarkable for the manner in which they illustrate the mode of formation of the tracks. These phalanges have become separated from the solid rock in which they were encased, so as to be removable at pleasure; and they thus show that the whole foot is not a simple impression in the rock which contains it, but a depression filled by foreign materials, i.e. by sand, clay, and other relics of pre-existing rocks. These materials had been gradually deposited in the mould formed by the bird's foot, and are therefore independent of this rock, in the same way as the plaster-of-Paris cast of a tooth, or any other body, is independent of the mould to which it owes its form. The impressions are in gray sandstone.
On the reversed surface of the slab is seen a small piece of broken quartz, about half an inch square. This piece forms a beautiful illustration of a part of the process by which the sandstone rocks are formed.
The second species of the same genus is the Brontozoum Sillimanium. Of this we have three specimens; the tracks have the same general character with the preceding, but are smaller.