OF THE BRITISH COAL MEASURES. 19 
ihe younger stems the parenchymatous strips are narrow; as the stem 
increased in thickness they widened out somewhat (Phot. 9). In one stem 
(specimen 5; P1.2. Phot. 13) the Sparganum structure is absent ; all the tissue 
of the outer cortex, so far as preserved, is thin-walled, and there is little sign 
of any differentiation. The stem is in an advanced stage of growth, so this 
peculiarity cannot be explained by youth; possibly this specimen may have 
been a rhizeme or underground portion of the stem. 
Beyond the Sparganum zone, where it is normally developed, we find two 
or three layers of cells larger than the sclerenchymatous fibres; they may 
have either thin or thick walls—the latter especially where they abut on 
sclerenchyma. In longitudinal section they are found to be elongated, but 
differ from the fibres in having transverse end-walls ; their contents are often 
carbonaceous. 
Lastly, we come to the epidermis, consisting of rather small and thick- 
walled cells, often remarkably well preserved. I have not, however, succeeded 
in detecting any stomata. 
We have now completed the description of the stem-structure, apart from 
the leaf-traces. At every point there are differences from the Lower 
Carboniferous H. Grievii, and when once the comparison is made no con- 
fusion between the species is possible. The relations of Æ. Lomaaii to the 
contemporary H. tiliwoides are much closer. 
The Leaj-traces. 
The distribution of the vascular bundles supplying the leaves is perhaps 
the most important point in the anatomy, and has not previously been 
investigated in H. Loma. The different specimens of the 8 form show 
some variations in this respect. We will begin with specimen 1, of which 
there is a series of 7 transverse sections; the order from below upwards 
appears to be as follows : 
B 447: 0.6; W.19150; Q 315 W. 1915; W.1915 B; W. 1915 A. 
There seems to be a gap between W. 1915 and W. 1915 D, but I believe 
that the same leaf-trace is present throughout the series. 
In the lowest section (8.447; P1.2. Phot. 9) there are two large bundles in the 
pericycle, 2:5 mm. apart *; each bundle causes a marked bulge in the pericycle ; 
on the inner side only one or two layers of cells invervene between the bundle 
and the ill-preserved phloem of the stele. No other leaf-trace is present, 
and the two strands are both in the same phase ; it therefore seems clear that 
both must belong to one and the same leaf-trace, for if they were two separate 
traces one would be further out than the other, and additional leaf-traces 
would be shown in other parts of the periphery. It might, indeed, be imagined 
that we have to do with a dorsiventral axis, the leaf-traces being given off 
from one side only as, e. g., in Helminthostachys ; in that case it is very unlikely 
* The clear interval is measured from xylem to xylem. 
