The formation of bird-tracks is well represented by a clay specimen, about an inch thick, and ten inches long. This is a piece of dried clay, obtained by President Hitchcock from the banks of the Connecticut, and produced by washings from clay on the shore above, covered with foot-impressions of a small tridactylous bird, and dried in the sun. This piece shows, in a way not to be questioned, the manner in which the ancient vestiges were produced. Sir Charles Lyell noticed a similar fact on the banks of the Bay of Fundy.
ORGANIC IMPRESSIONS.
The second great division of fossil impressions is called Organic, meaning impressions made by organized bodies; the bones of animals, fishes, and vegetables.
Near one extremity of the slab of the Ornithopus Gallinaceus is an elevation, about a foot long, and between one and two inches wide, projecting from the surface nearly half an inch. It has the appearance of a round bar of iron imbedded in the rock, which is clayey sandstone. This apparent bar of iron was probably a bone, buried in the stone, now silicified and impregnated with iron; the animal matter having entirely disappeared. In the slab of the Brontozoum Sillimanium is a projection about seven or eight inches long, and half an inch wide; probably the bone of an animal, perhaps a clavicle of the Brontozoum Giganteum.
The vestiges of fishes are very numerous in the sandstone rocks of Connecticut River. We have not less than two dozen specimens from this locality; a number equal to all the other specimens in our collection. These impressions of fishes are generally from three to six inches long, and three or four inches wide. They are of the grand division denominated by Professor Agassiz "heterocercal," having their tails unequally bilobed, from the partial prolongation of the dorsal spine; and they are considered to be of lower antiquity than the fishes which are entirely heterocercal. The most remarkable of the fish-specimens in our collection is a Cephalaspis (?): this fish is found in the specimen containing tracks of the Brontozoum Gracillimum, and traces of a turtle or tortoise. This fossil was discovered in the upper layer of the old red sandstone of Scotland, and had been mistaken by some for a trilobite: to us it appeared to be a Limulus, but further observation leads us to believe it to be a Cephalaspis. It exhibits a convex disc, four inches across, by two inches from above downwards, and a tail at right angles with the disc, the uncovered part of which is three inches long. The animal has been described by Professor Agassiz as being composed of a strong buckler, with a pointed horn at either termination of the crescent, and an angular tail.
To the vegetable impressions discovered among the sandstone rocks a peculiar name has not yet been assigned. When, however, we consider the strong probability that many impressions of stalks, leaves, fruits, and other parts of vegetables, may be hereafter discovered in these rocks, it will be found convenient to have a distinctive denomination. Vast numbers of vegetable impressions of a distinct and beautiful appearance, and in great variety, have been found in the coal-formation, which is nearly allied to the sandstone: such are the Sigillaria, Stigmaria, Equisetaceæ, Lycopodiaceæ, Coniferæ, Cycadeæ, &c. It is sufficient to say that the number of these has been already swelled to many hundreds: we must also believe, that some of the impressions in sandstone rocks which have been assigned to other substances ought to be attributed to vegetables. We may, therefore, venture to call the vegetable impressions "phytological."
A number of our slabs bear impressions of vegetables; either twigs of trees, or spires of plants. In a fragment broken from one of the toes of the Brontozoum Giganteum, we see a cylindrical depression, three inches long, and half an inch in diameter, marked by transverse lines, about the sixth of an inch apart, and presenting an unquestionable appearance of a fragment of a twig of an ancient vegetable, which had been trodden under the foot of the mighty Brontozoum. On the reversed surface of the same slab are found impressions, which were produced by a number of fragments of sticks, five or six inches long, lying at right angles, or nearly so. One of these sticks has been broken, and its pieces are slightly displaced from each other. Various other specimens contain the marks of sticks, or twigs of trees. The striæ, so distinctly discernable in a number of these portions, having been compared with twigs of the existing coniferæ (?), were found to resemble them. Some of these sticks show the appearance of incipient carbonization; yet the rock is sandstone, presenting, as already mentioned, distinct appearances of quartz, and other substances of which the arenaceous rocks are composed.