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.

Another very prominent member of the vegetation of the coal period, was that order of plants known as the Calamites.  The generic distinctions between fossil and living ferns were so slight in many cases as to be almost indistinguishable.  This resemblance between the ancient and the modern is not found so apparent in other plants.  The Calamites of the coal-measures bore indeed a very striking resemblance, and were closely related, to our modern horse-tails, as the equiseta are popularly called; but in some respects they differed considerably.