his period have
been obtained from such shallow-water deposits as the Old Red
Sandstone proper and the Gaspe series of North America, and few
traces of plant-life occur in the strictly marine sediments.
Apart from numerous remains, mostly of a problematical nature,
referred to the comprehensive group of the Sea-weeds, a large
number of Ferns have now been recognised, some being, of the
ordinary plant-like type (_Pecopteris, Neuropteris, Alethopteris,
Sphenopteris_, &c.), whilst others belong to the gigantic group
of the "Tree-ferns" (_Psaronius, Caulopteris_, &c.) Besides these
there is an abundant development of the singular extinct types of
the _Lepidodendroids_, the _Sigillarioids_, and the _Calamites_,
all of which attained their maximum in the Carboniferous. Of
these, the _Lepidodendra_ may be regarded as gigantic, tree-like
Club-mosses (_Lycopodiaceoe_); the _Calamites_ are equally gigantic
Horse-tails (_Equisetaceoe_); and the _Sigillarioids_, equally huge
in size, in some respects hold a position intermediate between
the Club-mosses and the Pines (Conifers). The Devonian rocks have
also yielded traces of many other plants (such as _Annularia,
Asterophyllites, Cardiocarpon_, &c.), which acquire a greater
pre-dominance in the Carboniferous period, and which will be
spoken of in discussing the structure of the plants of the
Coal-measures. Upon the whole, the one plant which may be considered
as specially characteristic of the Devonian (though not confined
to this series) is the _Psilophyton_ (fig. 77) of Dr Dawson.
These singular plants have slender branching stems, with sparse
needle-shaped leaves, the young stems being at first coiled up,
crosier-fashion, like the young fronds of ferns, whilst the old
branches carry numerous spore-cases. The stems and branches seem
to have attained a height of two or three feet; and they sprang
from prostrate "root-stocks" or creeping stems. Upon the whole,
Principal Dawson is disposed to regard _Psilophyton_ as a
"generalised type" of plants intermediate between the Ferns and
the Club-mosses. Lastly, the Devonian deposits have yielded the
remains of the first actual _trees_ with which we are as yet
acquainted. About the nature of some of these (_Ormoxylon_ and
_Dadoxylon_) no doubt can be entertained, since their trunks
not only show the concentric rings of growth characteristic of
exogenous trees in general, but their woody tissue exhibits under
the microscope the "discs" whi