[Illustration: FIG. 3.—Rhacopteris inaequilatera. Carboniferous limestone.]
Although many of the ferns greatly resembled existing species, yet there were others in these ancient days utterly unlike anything indigenous to England now. There were undoubted tree-ferns, similar to those which thrive now so luxuriously in the tropics, and which throw out their graceful crowns of ferns at the head of a naked stem, whilst on the bark are the marks at different levels of the points of attachment of former leaves. These have left in their places cicatrices or scars, showing the places from which they formerly grew. Amongst the tree-ferns found are megaphyton, paloeopteris, and caulopteris, all of which have these marks upon them, thus proving that at one time even tree-ferns had a habitat in England.
[Illustration: Fig. 4.—Frond of Pecopteris. Coal-shale.]
One form of tree-fern is known by the name of Psaronius, and this was peculiar in the possession of masses of aerial roots grouped round the stem. Some of the smaller species exhibit forms of leaves which are utterly unknown in the nomenclature of living ferns. Most have had names assigned to them in accordance with certain characteristics which they possess. This was the more possible since the fossilised impressions had been retained in so distinct a manner. Here before us is a specimen in a shale of pecopteris, as it is called, (pekos, a comb). The leaf in some species is not altogether unlike the well-known living fern osmunda. The position of the pinnules on both sides of the central stalk are seen in the fossil to be shaped something like a comb, or a saw, whilst up the centre of each pinnule the vein is as prominent and noticeable as if the fern were but yesterday waving gracefully in the air, and but to-day imbedded in its shaly bed.
[Illustration: FIG. 5.—Pecopteris Serlii. Coal-shale.]
Sphenopteris, or "wedge-fern," is the name applied to another coal-fern; glossopteris, or "tongue-leaf"; cyclopteris, or "round-leaf"; odonlopteris, or "tooth-leaf," and many others, show their chief characteristics in the names which they individually bear. Alethopteris appears to have been the common brake of the coal-period, and in some respects resembles pecopteris.
[Illustration: Fig. 6.—Sphenopteris Affinis. Coal-shale.]
In some species of ferns so exact are the representations which they have impressed on the shale which contains them, that not only are the veins and nerves distinctly visible, but even the fructification still remains in the shape of the marks left by the so-called seeds on the backs of the leaves. Something more than a passing look at the coal specimens in a good museum will well repay the time so spent.
What are known as septarian nodules, or snake-stones, are, at certain places, common in the carboniferous strata. They are composed of layers of ironstone and sandstone which have segregated around some central object, such as a fern-leaf or a shell. When the leaf of a fern has been found to be the central object, it has been noticed that the leaf can sometimes be separated from the stone in the form of a carbonaceous film.
Experiments were made many years ago by M. Goppert to illustrate the process of fossilisation of ferns. Having placed some living ferns in a mass of clay and dried them, he exposed them to a red heat, and obtained thereby striking resemblances to fossil plants. According to the degree of heat to which they were subjected, the plants were found to be either brown, a shining black, or entirely lost. In the last mentioned case, only the impression remained, but the carbonaceous matter had gone to stain the surrounding clay black, thus indicating that the dark colour of the coal-shales is due to the carbon derived from the plants which they included.