In the course of an extensive survey of the coal-field of South Wales, Mr. (now Sir William) Logan ascertained the important fact, that each coal-seam is underlaid by a bed of clay, in which the stems of stigmaria, branching freely in all directions, may be traced to the distance of many feet or even yards. They were recognised as undoubtedly occupying the site on which they grew, and consequently each coal-seam was held to rest upon an ancient soil. Some years afterwards, in making a cutting for the Lancaster and Bolton Railway, several upright massive stems belonging to a plant called _sigillaria_, were found to pass downwards into true stigmaria stems (Fig. 12). There could be no doubt that they were different parts of one and the same plant. This fact has since been abundantly demonstrated from the Nova Scotia coal-field. Many sigillariæ have been found there passing down into the fire-clay below, where they branch out horizontally as true stigmatiæ. It is evident, therefore, that the stigmaria was the under-ground portion of a plant, which, judging from the nature of the soil, and the free mode in which the tender rootlets branched off, appears to have lived in aquatic or marshy stations. [Illustration: Fig. 13. Stigmaria.] The stigmaria is too well marked to be readily confounded with any other coal-measure plant. It had a rounded stem, seldom more than four or five inches across, which was marked by a series of circular tubercules with a puncture in the centre, arranged in spiral lines round the stem. Each of these tubercules is surrounded, in ordinary specimens, by a circular depression,[21] and the whole plant (if one may use the comparison) looks as if it had been smitten with small-pox. From the hollow in the centre of each protuberance, there shot out a long round rootlet, formerly thought to be a leaf, and since the tubercules are pretty thickly set, the stigmaria must have had a somewhat hirsute appearance as it crept through the mud. It would resemble a thick bearded stem of ivy, save that the fibres, instead of running up two sides, were clustered all round it. Along the centre of the root, there ran a woody pith of a harder and more enduring texture than the surrounding part of the plant. The space between the outer tuberculed rind and the inner pith, seems to have been of a soft cellular nature, and to have decayed first, for the pith is sometimes hollow, and may not unfrequently be seen at a distance from the centre, and almost at the outer bark--a circumstance that seems only explicable on the supposition, that while the surrounding portions were decaying, the firmer pith altered its position in the hollow stem, sinking to the lower side, if the plant lay prostrate, and that it did not itself begin to decay until the interior of the stem had been at least partially filled up with sand or mud, or fossilized by the infiltration of lime. From the root of the sigillaria, which