742 PROFESSOR W. C. WILLIAMSON AND DR. D. H. SCOTT ON THE 
collection.* The meshes in this specimen are sometimes sub-divided by fine 
sclerenchymatous strands exactly as we find in the casts. The only “ Dictyoxylon” 
cortex, with which we are acquainted in Lyginodendron, was primary, We have 
good evidence that periderm was formed to the interior of this zone, and therefore it 
seems certain that, if the stem attained any large size, the characteristic cortical 
layer must have been cast off, and so could not have left the impressions which we 
see on the casts, We think, therefore, that no convincing evidence for the 
arborescent dimensions of Lyginodendron Oldhamium can be derived from the 
cortical impressions, 
There remains the large specimen showing structure, received from Mr, NEILD, and 
referred to in Memoir IV., p. 386. The specimen includes the pith and a portion 
of the wide zone of secondary wood. Sections in the three directions have been cut 
and clearly exhibit the structure so far as it is preserved.t The diameter of the pith 
is 8°3 X 2°3 centims. The maximum radial thickness of the secondary wood is 
5*8 centims., but we cannot be certain that its whole thickness is preserved. 
Assuming, however, that we have the whole thickness of the wood, the radius of the 
stem up to the cambium would have been over 7 centims., and its diameter over 
14 centims. We cannot tell what was the diameter of the whole stem, for we know 
nothing of the cortex. No authentic specimen of Lyginodendron which we have 
seen, however, has secondary wood of a greater thickness than about 6 millims.; so, 
if we judge by this dimension, the stem in question must have been nearly ten times 
as large as that of any undoubted Lyginodendron in the WILLIAMSON collection, 
Unfortunately, the only structure preserved is that of the secondary wood. Its 
general] anatomy is identical with that of certain specimens of Lyginodendron.t The 
tracheides are smaller than is usual in Lyginodendron, but not smaller than in 
some undoubted stems of that plant. The radial section shows the muriform rays 
and the pits on the walls of the tracheides, which, in so far as their preservation 
allows of comparison, agree very well with those of our plant. The pith is completely 
disorganized and no trace of the primary wood can be recognized. There is, however, 
a narrow incomplete zone of internal secondary wood, distinct from the rest, at the 
margin of the pith—which recalls the anomalous medullary tissue sometimes found 
in Lyginodendron. Although the parenchyma of the pith has perished, the cavity 
contains clusters of dark brown cells, which are much like the sclerotic nests 
characteristic of the pith m Lyginodendron. 
On the whole, until some other fossil has been found which agrees better with this 
doubtful stem, we think there is a presumption that it really belonged to a 
Lyginodendron, or to some plant of the same type of structure. 
* C.N. 665 and 668 (presented by M. Runavir). See also C.N. 703, figured in WHtuiamson, 
“ Organization,” Part IX., Plate 25, figs. 93-95. 
+ The sections are C.N. 1131, 1132, and 1133. 
t Of. especially C.N. 1124, 1176, and 1183. 
