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.