I NT K OD UCTION. 
xliii 
I 
definite places in the Order, seeing that the groups of living ferns depend upon the fructification, 
the venation being of very subordinate importance. Four leading types of venation have been 
recognised in the carboniferous ferns, and these constitute the four genera, or groups of genera, into 
which they are placed. These are — I. Neuropteris , containing the ferns in which there are numerous 
equal slender veins, dichotomously branching; II. Sphenopteris, having a midrib at the base of 
the pinnule, which either disappears altogether in the upper part, or is divided into uniform 
slender veins ; III. Pecopteris, including the ferns which have a distinct midrib passing through 
the pinnule to its apex, and giving off on either side the secondary veins ; and IV. Dictyopteris, 
containing eight or ten specific forms in which the veins are reticulated. These main divisions 
are again subdivided, but the smaller groups, based on the outline of the frond or the size and 
number of its divisions, cannot be recognised of generic value. 
The affinities of some of the carboniferous ferns with living plants have been more accurately 
determined in a few cases where some trace of the fructification has been preserved. The 
fronds of Pecopteris arborescens (Brongn.) frequently exhibit the impressions of the masses of 
sporangia. These are borne on the secondary veins in a single row on each side of the midrib. 
They form small rounded masses like the sori of Alsophila, but as the individual sporangia are 
not distinguishable, they may have been covered with an indusium like that of Cyathea. In 
other ferns an arrangement of the sporangia like what we have in Gleichenia has been observed ; 
in others again, ovoid sporangia crowned with a complete cap-shaped ring have been found, almost 
identical with the sporangia of Anemia , and arranged on the supporting pinnule in the same 
manner. And lastly, the cup-shaped involucres of the Filmy Fern, like those already described 
in Paleeopteris , have been noticed. An interesting confirmation of the presence of ferns belonging 
to the Hymenophyllece is obtained from the discovery of shed sporangia belonging to this group 
in the microscopic preparations made from nodules obtained from the coal at Oldham. These 
minute bodies are so well preserved that they exhibit the structure of the sporangium and 
its contents in as perfect condition as they are found in a recent plant. The elastic ring is 
oblique, and the pedicel is short and thick, as in the sporangia of the living species. 
The ferns are one of the least varying types of vegetable life. In the earliest ferns, 
where materials exist that enable an estimate to be made of their affinities, one is able to 
determine their places with such certainty and precision that they might be included in some 
existing genera. There existed, however, in the Carboniferous period, and survived through 
the newer Permian, a type of fern-stem which is now extinct. In the existing tree-ferns, 
as in some of the tree-ferns of the Coal period, the stem consists of a continuous vascular 
cylinder surrounding a pith or cellular axis, and pierced regularly with openings which connect 
the cellular tissue of the axis with that of the leaf, and give off from their everted margins the 
vascular bundles that go to the leaf. In the extinct type the vascular bundles which go 
to the leaves were formed free and complete in the cellular axis, and passed out entire, 
without touching the permanent tissues of the stem, through continuous longitudinal slits. 
These stems were generally surrounded with fleshy aerial roots, which have been well preserved 
in some specimens found in our English coal-fields, but more beautifully in silicified specimens 
from the Coal measures of Saxony and Bohemia. The stems of living tree-ferns, and those 
of the same type found in carboniferous rocks ( Caulopteris ), are analogous in the arrangement 
of their parts to what is found in the first year’s growth of a dicotyledon. In both there 
is a parenchymatous medulla, surrounded by a continuous vascular cylinder, which is 
perforated in a regular manner by meshes, for the passage out of the vascular elements to the 
appendages. The stems of the extinct group Stemmatoptcris have a structure analogous to 
