bark surrounding a zone principally composed of
"scalariform" vessels, this in turn enclosing a large central
pith. In their general appearance the _Lepidodendra_ bring to mind
the existing Araucarian Pines; but they are true "Cryptogams,"
and are to be regarded as a gigantic extinct type of the modern
Club-mosses (_Lycopodiaceoe_). They are amongst the commonest
and most characteristic of the Carboniferous plants; and the
majority of the "spore-cases" so commonly found in the coal appear
to have been derived from the cones of Lepidodendroids.

The so-called _Sigillanoids_, represented mainly by _Sigillaria_
itself (fig. 111), were no less abundant and characteristic of
the Carboniferous forests than the _Lepidodendra_. They commence
their existence, so far as known, in the Devonian period, but
they attain their maximum in the Carboniferous; and--unlike the
Lepidodendroids--they are not known to occur in the Permian period.
They are comparatively gigantic in size, often attaining a height
of from thirty to fifty feet or more; but though abundant and
well preserved, great divergence of opinion prevails as to their
true affinities. The _name_ of Sigillarioids (Lat. _sigilla_,
little seals or images) is derived from the fact that the bark
is marked with seal-like impressions or leaf-scars (fig. 111).

[Illustration: Fig. 111.--Fragment of the external surface of
_Sigillaria Groeseri_, showing the ribs and leaf-scars. The left-hand
figure represents a small portion enlarged. Carboniferous, Europe.]

Externally, the trunks of _Sigillaria_ present strong longitudinal
ridges, with vertical alternating rows of oval leaf-scars indicating
the points where the leaves were originally attached. The trunk
was furnished with a large central pith, a thick outer bark,
and an intermediate woody zone,--composed, according to Dawson,
partly of the disc-bearing fibres so characteristic of Conifers;
but, according to Carruthers, entirely made up of the "scalariform"
vessels characteristic of Cryptogams. The size of the pith was
very great, and the bark seems to have been the most durable
portion of the trunk. Thus we have evidence that in many cases
the stumps and "stools" of _Sigillarioe_, standing upright in
the old Carboniferous swamps, were completely hollowed out by
internal decay, till nothing but an exterior shell of bark was
left. Often these hollow stumps became ultimately filled up with
sediment, sometimes enclosing the remain