tomy of the stem is simple
and root-like; the cones are remarkable for the fact that each scale or
sporophyll is a double structure, consisting of a lower, usually sterile
lobe and one or more upper lobes bearing the sporangia; in one species
both parts of the sporophyll were fertile. Sphenophyllum was evidently
much specialised; the only other known genus is based on an isolated
cone, Cheirostrobus, of Lower Carboniferous age, with an extraordinarily
complex structure. In this genus especially, but also in the entire
group, there is an evident relation to the Equisetales; hence it is of
great interest that Nathorst has described, from the Devonian of Bear
Island in the Arctic regions, a new genus Pseudobornia, consisting of
large plants, remarkable for their highly compound leaves which,
when found detached, were taken for the fronds of a Fern. The whorled
arrangement of the leaves, and the habit of the plant, suggest
affinities either with the Equisetales or the Sphenophyllales; Nathorst
makes the genus the type of a new class, the Pseudoborniales. (A.G.
Nathorst, "Zur Oberdevonischen Flora der Baren-Insel", "Kongl. Svenska
Vetenskaps-Akademiens Handlingar" Bd. 36, No. 3, Stockholm, 1902.)

The available data, though still very fragmentary, certainly suggest
that both Equisetales and Sphenophyllales may have sprung from a
common stock having certain fern-like characters. On the other hand the
Sphenophylls, and especially the peculiar genus Cheirostrobus, have in
their anatomy a good deal in common with the Lycopods, and of late years
they have been regarded as the derivatives of a stock common to
that class and the Equisetales. At any rate the characters of the
Sphenophyllales and of the new group Pseudoborniales suggest the
existence, at a very early period, of a synthetic race of plants,
combining the characters of various phyla of the Vascular Cryptogams.
It may further be mentioned that the Psilotaceae, an isolated epiphytic
family hitherto referred to the Lycopods, have been regarded by several
recent authors as the last survivors of the Sphenophyllales, which they
resemble both in their anatomy and in the position of their sporangia.

The Lycopods, so far as their early history is known, are remarkable
rather for their high development in Palaeozoic times than for any
indications of a more primitive ancestry. In the recent Flora, two
of the four living genera (Excluding Psilotaceae.) (Selaginella and
Isoetes) h