water the stems of laminaria. When highly magnified, a mesial groove might be detected running along each of the hair-like lines. With these marine plants we occasionally find large rectilinear stems, resolved into a true coal, but retaining no organic character by which to distinguish them. As I have seen some of these more than three inches in diameter, and, though existing as mere fragments, several feet in length, they must, if they were also plants of the sea, have exceeded in size our largest laminaria.[46] And such are the few vegetable organisms, of apparently aquatic origin, which I have hitherto succeeded in detecting in the Lower Old Red Sandstone of Scotland.[47] Their individual numbers, however, must have been very great, though, from the destructible character of their tissues, their forms have perished in the stone. The immensely developed flagstones of Caithness seem to owe their dark color to organic matter mainly of vegetable origin. So strongly bituminous, indeed, are some of the beds of dingier tint, that they flame in the fire like slates steeped in oil. [Illustration: Fig. 120.] The remains of a terrestrial vegetation in this deposit are greatly scantier than those of its marine plants; but they must be regarded as possessing a peculiar interest, as, with the exception of the spore cases of the Ludlow rocks, the oldest of their class, in at least the British islands, whose true place in the scale can be satisfactorily established. In the flagstones of Orkney there occurs, though very rarely, a minute vegetable organism, which I have elsewhere described as having much the appearance of one of our smaller ferns, such as the maidenhair-spleenwort, or dwarf moonwort. It consists of a minute stem, partially covered by what seems to be a small sheath or hollow bract, and bifurcates into two fronds or pinnæ, fringed by from ten to twelve leaflets, that nearly impinge on each other, and somewhat resemble in their mode of arrangement the leaflets of one of our commonest Aspleniums,--_Asplenium trichomanes_. One of our highest authorities, however, in such matters (Professor Balfour of Edinburgh) questions whether this organism be in reality a fern, and describes it from the specimen on the table, in the Palæontological chapter of his admirable Class Book, simply as "a remarkable pinnate frond." (Fig. 13, p. 56.) We find it associated with the remains of a terrestrial plant allied to lepidodendron, and which in size and general appearance not a little resembles one of our commonest club mosses,--_Lycopodium clavatum_.[48] It sends out its branches in exactly the same style,--some short and simple, others branched like the parent stem,--in an arrangement approximately alternate; and is everywhere covered, stem and branch, by thickly set scale-like leaflets, that, suddenly narrowing, terminate in exceedingly slim points. It has, however, proportionally a stouter stem