al-measures are most striking, and point to a time when the sandy clay
which imbedded them was borne by water in a very tranquil manner, to be
deposited where the ferns had grown, enveloping them gradually, and
consolidating them into their mass of future shale. In one species known
as the _neuropteris_, the nerves of the leaves are as clear and as
apparent as in a newly-grown fern, the name being derived from two Greek
words meaning "nerve-fern." It is interesting to consider the history of
such a leaf, throughout the ages that have elapsed since it was part of a
living fern. First it grew up as a new frond, then gradually unfolded
itself, and developed into the perfect fern. Then it became cut off by
the rising waters, and buried beneath an accumulation of sediment, and
while momentous changes have gone on in connection with the surface of
the earth, it has lain dormant in its hiding-place exactly as we see it,
until now excavated, with its contemporaneous vegetation, to form fuel
for our winter fires.

[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