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XII. On the Organisation of the Fossil Plants of the Coal-Measures. — Part XITI. 
Heterangium Tiliaeoides (Williamson) and Kaloxylon Hookeri. 
By W. C. Williamson, LL.D., F.R.S., Professor of Botany in the Owens College 
and in the Victoria University. 
Received December 1, 1886,—Read January 6,—Revised May, 1887. 
[Plates 21—24.] 
In 1872 I described (‘ Phil. Trans.,’ 1873, “ On the Organisation of the Fossil Plants 
of the Coal-Measures—Part IV.”), under the name of Heterangium Grievii, one of 
the most interesting of the various plants which I discovered in the Burntisland 
deposit of Carboniferous limestone at Kinghorn. More recently we have obtained, 
from the Halifax beds, a very distinct plant, which, though differing in many 
important features from II. Grievii, resembles it so closely in others that I propose to 
include it in the same genus, under the name of Heterangium Tiliaeoides. Its central 
medullary axis, A, differs in no conspicuous manner from that of II. Grievii when 
transverse sections of the two are compared. 
Fig. 1 represents such a section enlarged 15 diameters, the medullary portion being- 
seen at A, and a similar section is shown at fig. 2, A ; whilst fig. 3 represents a 
small portion of a section enlarged 103 diameters. From the latter of these sections 
it will be seen that this medullary axis consists of irregular clusters of vessels or 
tracheids, h, the intervals between which are filled with ordinary parenchyma, a. 
The vessels near the periphery of this axis are smaller than those constituting the 
more central bundles—a feature best shown in fig. 2, h'. The character of these 
vessels will be referred to immediately. 
Closely surrounding the medullary axis is a well-defined exogenous zone, B. 
This consists of a circle of vascular wedges, c ; each wedge being composed of a 
number of laminae which spring from a cluster of the smaller vessels, //, which form 
the periphery of the medullary axis. At their medullary extremities the more external 
of the laminae, fig. 4, d, composing each wedge, bend inwards towards its more central 
ones, so that each bundle represents an obtuse wedge-shaped group of such laminae 
separated from a similar group on either side of it by a primary medullary ray 
(figs. 1, 2, and 4, g). The vessels of each lamina increase in size from within 
outwards, and between each one or two laminae (fig. 4, d) we find secondary medullary 
MDCCCLXXXVII.—B. 2 P 30.9.87 
