FOSSIL FERNS. Lt 
scars left by the separation of the petioles ; and internally 
by that peculiar zone, formed of bundles of ligneous tissue 
inclosed in sheaths, which encircles the central axis, as 
shown in the transverse sections in Lign. 2, ante, p.62. The 
leaves may be identified by the form of their segments, 
which are disposed with remarkable regularity, and have a 
peculiar mode of subdivision ; and above all, by the delicacy, 
evenness, and distribution of the veins. There are upwards 
of two thousand species of living ferns, and in the fossil 
kingdom the number is considerable ; more than two hun- 
dred have been collected from the carboniferous formation. 
The recent tree-ferns are confined almost exclusively to the 
equinoctial regions ; humidity and heat being the conditions 
most favourable to their development (Vég. Foss. p. 141. 
Bd. p. 461. Wond. p. 727). 
From the elegance and diversity of form of their foliage, 
fossil ferns are the most remarkable and attractive vegetable 
remains in the ancient strata. The greater number are from 
the coal deposits, the fern-leaves generally occurring in the 
schists or shales that form the roof of the beds of coal. 
Many of the strata are made up of carbonized fern-leaves 
and stems closely pressed together. The roof of a coal 
mine, when newly exposed, often presents a most interesting 
appearance, from the abundance and variety of leaves, 
branches, and stems, that occur either in relief, or impressed 
on the dark glossy surface. The specimens selected for 
illustration exhibit the principal modes of venation on which 
the genera are founded. 
The fossil genera have been established by M. Ad. Brong- 
niart, from the form of the leaves and the characters of their 
venation ; that is, the distribution of the vessels. In the 
following descriptions some botanical phrases are necessarily 
employed ; a few terms of frequent occurrence are explained 
in Lign. 16. 
