334 
PEOFESSOE W. G. WILLIAMSON ON THE OEGANIZATION 
is nearly 24 inches in breadth and perfectly flat. Hence it is obvious that all these 
specimens belonged to stems or branches of arborescent dimensions, whilst the successive 
gradations in the size of their areolae clearly indicate that their differences are merely 
those of age and growth. Specimens in my own cabinet, and a sharply defined one in 
that of Mr. Boyd Dawkins, link the above series with those described in the preceding- 
pages, leaving no room for doubting that the whole belong to one form of vegetation, 
of which the larger examples were casts of the bark of the arborescent stems, and the 
smaller ones, in which the entire organization is preserved, the peripheral extremities 
or branches. The explanation of these casts is sufficiently obvious. The fibrous bands, 
forming a coarse network in Plate XXIV. figs. 13 & 15, have constituted a firm frame- 
work, alike resisting atmospheric abrasion and contraction through desiccation. The 
enclosed cellular areoke have been characterized by precisely opposite features. Hence, 
where the latter reached the surface of the bark, deep lenticular depressions presented 
themselves, whilst the surrounding fibrous bands stood up in bold relief. That these 
results were partly due to the shrinking of the cellular areoke is shown by two specimens 
in my cabinet : they both consist of the usual prosenchymatous bark of branches in 
which the inner or parenchymatous layer has been replaced by inorganic matter ; but 
the inner surface of the outer bark exhibits precisely the same areolation, save that the 
concavities of the areola; are now directed inwards instead of outwards , that I have just 
described as characterizing the periphery of the same bark-layer. The difference in the 
relative size of the areolations and of the intervening flat spaces in Plate XXVII. figs. 26 
& 28 has obviously been due to corresponding differences in the thickness of the raised 
fibrous bands in relation to the size of the cellular areolae which they enclosed. We thus 
learn that whilst Lyginodendron is undoubtedly an inorganic cast of the prosenchymatous 
layer of the bark of Dictyoxylon , it may either represent its exterior surface, which has 
impressed its contour upon the surrounding mud or sand, or it may represent its inner 
surface, to which the inorganic material has obtained access through the accidental 
destruction of the inner parenchyma of the bark. Combining these observations with 
tire evidence derived from Mr. Nield’s large specimen of the ligneous zone of Dicty- 
oxylon Oldhamium, described on page 386, we are irresistibly brought to the conclusion 
that Dictyoxylon Oldhamium was an arborescent tree, which, from the comparative 
abundance of its fragments preserved in the Lower Coal-measures of Lancashire, must 
have formed one of the most conspicuous features of the forests of the Carboniferous age. 
The bearing of the facts just recorded upon nomenclature will be considered after 
describing the next subject of this memoir. 
In the spring of 1871, Dr. Dawson, of Montreal, conferred upon me the double 
favour of directing my attention to the plants of Burntisland, and of introducing to me 
their energetic discoverer, G. Grieve, Esq. I soon found amongst the rich treasures 
which that gentleman had brought to light a most remarkable plant, closely allied in 
many points of its structure to Dictyoxylon Oldhamium. I gave a very brief and hasty 
description of this plant at the Edinburgh Meeting of the British Association, assigning 
