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154 ON THE GENUS KNORTIIA.
truncate processes, supposed to be fleshy leaves, are present, but in
the upper portion these are exhibited as long, slender processes, IVom
two to three inches long. They are composed of the same amorphous
sand Avhich forms the stem itself, and are consequently casts proceeding
from, and filled up through the stem. These processes are all free from
the stem, being separated from it by a tliin film of coal, and in con-
sequence of this, tlie processes are broken off from the lower portion
of the specimen. The whole stem is covered with a thin layer of coal,
which separates it from the incrusting rock.
In examining the structure of the stem of Lepidodendron we find, as
already stated, that the woody cylinder is too slender to have formed
the mould in which Knorria was cast. The wood was surroiuided by
a cellular tissue of considerable thickness, and so delicate that it has
never yet been seen preserved in anything like its entirety. It is
generally replaced by some amorphous or crystalline substance, and its
nature has been detected only by the occasional preservation of small
portions, Avhich have been protected by their neighbourhood to the
woody cylinder, or to the outer sub-cortical layer. This outer layer is
composed of small, regularly arranged, elongated cells. It appears to
have been more durable than any of the other tissues, having resisted
the decay which speedily destroyed the medulla, and the delicate cel-
lular structure between it and the wood, and even the woody cylinder
which, from its relation to these two cellular structures, was probably
more liable to decay. The specimens of erect Sigillari(S, discovered by
Mr. Wiinsch, in Arran, preserved erect in beds of volcanic ash, are com-
pletely hollowed out ; all the interior cellular and vascular tissue has
disappeared, and only the layer of elongated cells and the outer cortical
layer of indurated cells remain. This compact cylinder of elongated
cells in Lepidodendron is penetrated in a spiral manner by the vascular
bundles which pass to^xthe leaves. These bundles are composed of a
few scalariform vessels, surrounded by a considerable quantity of cel-
lular tissue, of the same delicate structure as the inner layer which
is always altogether, or almost altogether absent, and, like it also, it is
very rarely preserved.
This specimen then shows that Knorria is a Lepidodendroid stem
which, after being imbedded in mud or sand, lost by decay the whole
of its interior up to the cylinder of elongated cells, and lost besides
this, the vascular bundles with the accompanying cellular tissue which