I have only to add to this description that when we consider that _Nematophyton Logani_ was a large tree, sometimes attaining a diameter of more than two feet, and a stature of at least twenty before branching; that it had great roots, and gave off large branches; that it was an aërial plant, probably flourishing in the same swampy flats with _Psilophyton_, _Arthrostigma_, and _Leptophleum_; that the peculiar bodies known as Pachytheca were not unlikely its fruit--we have evidence that there were, in the early Palæozoic period, plants scarcely dreamt of by modern botany. Only when the appendages of these plants are more fully known can we hope to understand them. In the mean time, I may state that there were probably different species of these trees, indicated more particularly by the stems I have described as _Nematoxylon_ and _Celluloxylon_[AH] There were, I think, some indications that the plants described by Carruthers as _Berwynia_, may also be found to have been generically the same. The resinous matter mentioned by Prof. Penhallow is found in great abundance in the beds containing _Nematophyton_, and must, I think, have been an exudation from its bark. [AH] "Journal Geol. Society of London," 1863, 1881. CHAPTER III. THE ERIAN OF DEVONIAN FORESTS--ORIGIN OF PETROLEUM--THE AGE OF ACROGENS AND GYMNOSPERMS. In the last chapter we were occupied with the comparatively few and obscure remains of plants entombed in the oldest geological formations. We now ascend to a higher plane, that of the Erian or Devonian period, in which, for the first time, we find varied and widely distributed forests. The growth of knowledge with respect to this flora has been somewhat rapid, and it may be interesting to note its principal stages, as an encouragement to the hope that we may yet learn something more satisfactory respecting the older floras we have just discussed. In Goeppert's memoir on the flora of the Silurian, Devonian, and Lower Carboniferous rocks, published in 1860,[AI] he enumerates twenty species as Silurian, but these are all admitted to be Algæ, and several of them are remains which may be fairly claimed by the zoologists as zoophytes, or trails of worms and mollusks. In the Lower Devonian he knows but six species, five of which are Algæ, and the remaining one a _Sigillaria_, but this is of very doubtful nature. In the Middle Devonian he gives but one species, a land-plant of the genus _Lepidodendron_. In the Upper Devonian the number rises to fifty-seven, of which all but seven are terrestrial plants, representing a large number of the genera occurring in the succeeding Carboniferous system. [AI] Jena, 1860.