In the fourth of this series of Memoirs (‘Phil. Trans.,' 1873, p. 377, et seq .) I described a remarkable plant under the name of Dictyoxylon Oldhamium ; I also gave reasons for substituting the late Mr. Gourlie ’s generic name of Lyginodendron for that of Dictyoxylon . In the same Memoir (p. 403) I referred to some petioles, to which I proposed to assign the name of Edraxylon ; but later researches demonstrated the necessity for abandoning this as a generic term and substituting for it the more comprehensive one of Rachiopteris aspera . In my Memoir, Part VI. ('Phil. Trans.,' 1874, Plate 2, p. 679, et seq .), I described this proposed Edraxylon under the name of Rachiopteris aspera . Certain similar features exhibited by the above two plants led me to remark in Memoir IV., p. 403, after showing that the Rachiopteris aspera was obviously the petiole of a Fern, “I think it far from impossible that these may prove to belong to Dictyoxylon ( Lyginodendron ) Oldhamium ; but since I have not yet succeeded in correlating them with any certainty, 1 shall add no more respecting them at present.” Since 1873 1 have accumulated a vast amount of material illustrative of the structure and relations of these two plants, and am now in a position to demonstrate that they respectively represent the stem and petiole of the same organism which proves to be a Fern. I was long under the conviction that the remarkable exogenous development of the stems of many of the Carboniferous Cryptogams, which I have so continuously demonstrated to exist, and which is now so universally recognised by Palæontologists, had no existence amongst Ferns. I have now to show that this development did exist amongst Ferns as well as amongst the arborescent Lycopods and Calamites, in which it is so conspicuous. Fig. 1 (Plate 12) is part of a transverse section of a stem or branch of Lyginodendron Oldhamium , in which a represents the medulla; b , the exogenous xylem zone; c , the place of the inner cortex, wanting in this specimen; d , one of the pairs of vascular bundles, so characteristic of the, cortex of this plant; e , the outermost cortex, composed, in transverse sections, of radiating bands of sclerenchyma, g , alternating with parenchymatous areas, f . At k, k we find two bundles of tracheids, like those at d , forming the centre of the cortical structures of a petiole of Rachiopteris aspera , i, i , which petiole is organically united to the cortex e of the Lyginodendron . The two bundles k, k are assuming the oblique relative positions seen in the similar bundles of the free petiole of R. aspera , represented in fig. 2. Other sections in my cabinet, similar to fig. 1, demonstrate the same facts, viz., that the pairs of bundles, fig. 1, d , which form so characteristic a feature of transverse sections of the middle cortex of Lyginodendrom Oldhamium , pass outwards, through the outer cortex, to become the tracheæal bundles of the petioles of the plant, and which petioles I had previously designated Rachiopteris aspera . I may state that my friend Graf Solms-Laubach, who has obtained numerous specimens of the Lyginodendron associated with others of Rachiopteris aspera from a locality on the continent, agrees with me in the conclusion at which I have arrived respecting their unity. The more perfect specimens of the Lyginodendron obtained during the last seventeen years have thrown yet further light upon those figured in 1873. In the latter, as at fig. 1, c, no traces of the middle bark were preserved; but examples from Halifax, for which I am indebted to my friends Mr. Cash and Mr. Spencer, of Halifax, have supplied what was wanting. Fig. 3 is a transverse section in which this inner cortex, c , is shown to consist of a zone of extremely delicate, thin-walled parenchymatous cells, scattered throughout which are numerous gum-canals, l . Three of these canals are represented, enlarged 250 diameters, in figs. 4 and 5, embedded in the thin-walled cells, c, c , of the cortex.