494 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
merely results of the expanding growth of the inner portions of the stem Assuring the 
outer bark, as is the case with exogenous stems. 
I may here express my conviction that many of the trees that have been loosely 
designated Sigillarian have no claim to be regarded as such. Dr. Dawson and other 
palaeontologists have recorded the fact that the characteristic leaf-scars and other 
superficial features of the younger stems and branches disappear towards the base of 
the older trunks ; and this has evidently been the case with the Arran specimens. 
Our prolonged researches failed to supply the smallest portion of a true Sigillaria. 
I obtained one fragment of a cast of the outer surface of a bark exhibiting what at the 
first glance had a Sigillarian appearance. It exhibits strongly-marked ridges, but 
these proved to be but casts of the unsymmetrical longitudinal fissures just referred 
to. That such is the case is shown by the positions of the oblong leaf-scars—which are 
wholly independent of those of the ridges—which could not have been the case in a 
Sigillaria, These leaf-scars are of the Lepidodendroid type. 
The most numerous fragments which we met with were long, slender Lepidodendroid 
twigs, densely clothed with very short, scaly leaves, such twigs being usually about 
half an inch in diameter; fragments of larger branches were not uncommon, some of 
these being from two to three inches in diameter, and which were also true Lepido- 
dendra, retaining the closely-united, rhomboidal bases of their leaves in union with the 
outer bark, each rhomboid having a diameter of a quarter of an inch. Mr. Binney 
has figured and described some fruits found by Mr. Wunsch, all of which are true 
Lepidostrobi furnished with macrospores and microspores ; and I am indebted to 
Dr. Young and to Mr. John Young, of the Glasgow University, for their permission 
to make a section of a similar Lepidostrobus from Laggan Bay, but which unfortu¬ 
nately contained no spores. We have thus a mass of evidence of a positive kind 
indicating that these stems were true Lepidodendra and not Sigillarice —a conclusion 
which the negative testimony afforded by the entire absence of true Sigillarian frag¬ 
ments from these beds equally sustains. Most of these specimens apparently belonged 
to one species; only the one fragment of a cast of the bark already referred to appears 
to have been distinct from the rest. 
Fig. 1 is a transverse section of one of the small twigs that abound in the deposit 
in such numbers as to leave no room for doubting that they are the ultimate branches 
of the large stems. These twigs are long and slender, generally about half an inch in 
extreme diameter, including the closely-compressed scale-like leaves. In the centre of 
the section is the principal vascular bundle, a, of which a more enlarged representation 
is given in fig. 2 ; it is composed entirely of barred vessels, without any visible inter¬ 
mixture of cellular tissue. The vessels are pretty uniform in size, the largest being 
about '005 in diameter*; but at the periphery of the bundle we find a limited number 
of much smaller vessels. It is from these latter that are derived the numerous foliar 
vascular bundles (fig. 2, b) that cluster round the mam axis. The inner portions of 
* These measurements are recorded, as in my previous memoirs, in the decimal parts of an inch. 
