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 remains of galley-worms, land-snails, or Amphibians, which formerly found in the cavity of the trunk a congenial home; and from the sandstone or shale now filling such trunks some of the most interesting fossils of the Coal-period have been obtained.  There is little certainty as to either the leaves or fruits of Sigillaria, and there is equally little certainty as to the true botanical position of these plants.  By Principal Dawson they are regarded as being probably flowering plants allied to the existing “false palms” or “Cycads,” but the high authority of Mr Carruthers is to be quoted in support of the belief that they are Cryptogamic, and most nearly allied to the Club-mosses.

[Illustration:  Fig. 112.—­Stigmaria ficoides.  Quarter natural size.  Carboniferous.]

Leaving the botanical position of Sigillaria thus undecided, we find that it is now almost universally conceded that the fossils originally described under the name of Stigmaria are the roots of Sigillaria, the actual connection between the two having been in numerous instances demonstrated in an unmistakable manner.  The Stigmarioe (fig. 112) ordinarily present themselves in the form of long, compressed or rounded fragments, the external surface of which is covered with rounded pits or shallow tubercles, each of which has a little pit or depression in its centre.  From each of these pits there proceeds, in perfect examples, a long cylindrical rootlet; but in many cases these have altogether disappeared.  In their internal structure, Stigmaria exhibits a central pith surrounded by a sheath of scalariform vessels, the whole enclosed in a cellular envelope.  The Stigmarioe are generally found ramifying in the “under-clay,” which forms the floor of a bed of coal, and which represents the ancient soil upon which the Sigillarioe grew.

[Illustration:  Fig. 113.—­Trigonocarpon ovatum.  Coal-measures, Britain. (After Liudley and Hutton.)]