t 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 sp