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.]