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 t