[Illustration:  Fig. 110.—­Lepidodendron Sternbergii, Carboniferous, Europe.  The central figure represents a portion of the trunk with its branches, much reduced in size.  The right-hand figure is a portion of a branch with the leaves partially attached to it; and the left-hand figure represents the end of a branch bearing a cone of fructification.]

The Lepidodendroids, represented mainly by the genus Lepidodendron itself (fig. 110), were large tree-like plants, which attain their maximum in the Carboniferous period, but which appear to commence in the Upper Silurian, are well represented in the Devonian, and survive in a diminished form into the Permian.  The trunks of the larger species of Lepidodendron at times reach a length of fifty feet and upwards, giving off branches in a regular bifurcating manner.  The bark is marked with numerous rhombic or oval scars, arranged in quincunx order, and indicating the points where the long, needle-shaped leaves were formerly attached.  The fruit consisted of cones or spikes, carried at the ends of the branches, and consisting of a central axis surrounded by overlapping scales, each of which supports a “spore-case” or seed-vessel.  These cones have commonly been described under the name of Lepidostrobi.  In the structure of the trunk there is nothing comparable to what is found in existing trees, there being a thick 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.]