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IV. On the Organisation of the Fossil Plants of the Coal-Measures. —Part XVII. 
By Wm. C. Williamson, LL.D.. F.R.S., Professor of Botany in the Owens College, 
Manchester. 
Received February 8,—Read March 13, 1890. 
[Plates 12-15.] 
Lyginoclendron Oldhamium and Rucldopteris cis-pera. 
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 Oldhamiimi; I also gave reasons 
for substituting the late Mr. Gourlie’s geneiic 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 sulistituting for it the more com¬ 
prehensive one of Racliiopteris. In my Memoir, Part VI. (‘Phil. Trans.,’ 1874, 
Plate 2, p. 670, et seq.), I described this proposed Edraxylon under the name of 
Rachioj)teris aspera. Certain similar features exhibited by the above two plants led 
me to remark in Memoir IV., p. 403, after showing tha-t the Racliiopteris aspera was 
obvioirsly the })etiole of a Fern, “ I think it far from impossilde that these may prove 
to belong to Dictyoxylon i^Lyginodendron) Oldhamium-, Init since I have not yet 
succeeded in correlating them with any certainty, I shall add no more respecting 
them at present.” 
Since 1873 I 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 
Palaeontologists, 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 ; h, the exogenous xylera zone ; c, the place of the inner cortex, wanting in 
this specimen ; cl, one of the pairs of vascular bundles, so characteristic of the cortex 
MDCCCXC.—B. N 10.11.90. 
