50G 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
For specimens of a second and more remarkable Rachiopteride I am indebted to 
both Mr. Binns and Mr. Spencer. They are from the Halifax beds. Fig. 19 repre¬ 
sents a transverse section of Mr. Binns’ specimen enlarged 15 diameters, and fig. 20 
represents the central portion of fig. 19, enlarged 46 diameters. The outer cortex, a, 
is identical in all respects with the corresponding tissue in the other Bachiopterides 
that I have described, consisting of a thick-walled cellular prosenchyma, the peripheral 
cells of which are much smaller than those of its inner portion. Within this is a more 
delicate thin-walled parenchyma, b, the transition from which to the investing pro¬ 
senchyma is somewhat abrupt. This inner tissue is frequently wanting in these 
Rachiojpteride sections, it having apparently disappeared through decay, prior to the 
mineralisation of these stems. The yet more internal structures are best illustrated by 
the enlarged fig. 20, in which b again represents the parenchyma of the middle cortex. 
Within this we find a narrow zone, of inner cortex c, the cells of which exhibit a 
decided tendency to arrange themselves in regular radiating lines. This tissue passes 
into a still more internal series of cells, d, which fill up the interspaces and round off 
the outlines of the vascular bundle. Between the two series of cells, c and d, a dark, 
ill-defined, but nevertheless distinct line of demarcation, h, surrounds the entire central 
vasculo-cellular axis. The vessels, e, f, are arranged in this plant as in the genus 
Zijgopteris of authors. The central ones, e, are extremely large, some of them 
having a diameter of '01. These are virtually arranged in two rows, which separate 
at each end of the bundle, where the vessels rapidly diminish in size, and recombine 
to form the two vascular loops, f, f, enclosing a small mass of parenchyma, g, g, located 
at each extremity of the double row of large vessels. Many of the vessels of these 
peripheral loops, f are not more than ’0012 in diameter, but, like the larger ones, 
they are thick-walled. The tissues d and g probably represent liber structures. 
All the larger vessels of this bundle are densely crowded with tylose-cells, consti¬ 
tuting the second example which I have met with of this tissue existing in carboni¬ 
ferous plants. I described a previous one in the case of the fern-like Rachiopteris 
corrugata in my memoir, Part VIII.* 
Fig. 21 represents the central part of a transverse section of a similar stem to 
fig. 19, but made so obliquely as to be almost a longitudinal one. From it we learn 
that all the cells c and d enclosed within the middle cortical layer b, b, excepting g, 
are long, narrow, and have square ends. We also see that the large vessels, e, and 
the small ones, f are alike barred, the former being filled with tylose as in the 
transverse section. 
Fig. 22 represents the transverse section of another stem, for which I am indebted 
to Mr. Binns. It corresponds with fig. 20 in almost every essential respect, but the 
large vessels, e, exhibit no traces of tylose. The cells of the innermost portions, 
c, of the cortical layer exhibit less tendency to arrange themselves in radiating 
lines than they do in fig. 20, and the dark line, h, separating that layer from the 
* Phil. Trans., Yol. 167, Part I., p. 214, Plate 6, figs. 15 and 16. 
