228 
PROFESSOR AV. C. WILLIAMSON ON THE ORGANIZATION 
possessed similar ones to those which Witiiam and I have figured. Further, the descrip- 
tion which M. Beongniaet has given of the structure of the outer bark and epiderm of 
his plant, these being the only cortical elements remaining in his specimen, would apply 
with little or no alteration to several of my Lepidodendroid and Sigillarian types ; so that 
whilst a really indisputable Sigillaria, like my Plate XXIX. fig. 39, but in which the 
woody axis is preserved in situ, is still an important desideratum, I have very little doubt 
that, when discovered, it will be found to correspond with one of the several varieties of 
Digloxylon. Most probably also my Plate XXV. fig. 8, representing one of the extreme 
of the two types figured by Mr. Binney under the name of Sigillaria vascularis, will also 
be found to belong to the same subtype of the same genus. Yet my indefatigable friend 
informs me that his cabinet contains specimens in which the most gradual transition can 
be traced from the plant just referred to to the Lejndodendronselaginoides, the oppositely 
divergent form of the same group ; hence his inclusion of both under one common 
name. 
Of the form recently described by Dr. Dawson* I know nothing, having seen nothing 
like it amongst our Lancashire Coal-measures. He describes a coniferous type of glan- 
dular prosenchyma as occurring in the woody axis of his Sigillaria . I have not seen a 
single fibre of this kind in any of our Sigillarian or Lepidodendroid forms, neither have 
I met with any trace of a Sternbergian pith such as he describes in the same plant, 
which evidently belong to a different type from those of our English Coal-measures, 
assuming it to be what Dr. Dawson supposes, viz. a true Sigillaria. 
If, then, I am correct in thus bringing the Lepidodendra and Sigillarise into such close 
affinity, there is an end of M. Beongniakt’s theory, that the latter were Gynmospermous 
Exogens, because the Cryptogamic character of the former is disputed by no one ; we 
must rather conclude, as I have done, that the entire series represents, along with the 
Catamites, an exogenous group of Cryptogams in which the woody zone separated a 
medullary from a cortical portion. The Cryptogamic type of structure remains in the 
universal, if not even exclusive, prevalence of barred vessels, a modification of that 
iscalariform type so characteristic of living Cryptogams. The medulla in some cases fails 
to attain to the simple parenchymatous condition common amongst Exogens ; nor does the 
hark, as already observed, exhibit the division into epiphloeum, mesophlceum, and endo- 
phlceum. But neither can these divisions be traced in the Cycads, with which, in some 
respects, the carboniferous stems exhibit remarkable affinities. 
The semivascular bast-layer of the epiderm of these Lepidodendroid and Sigillaroid 
plants has played an important part in their preservation ; it has arrested the decay 
which appears to have usually commenced in the inner bark, simultaneously perhaps 
with that of the cells of the medulla, though the latter not unfrequently remain after 
the former have disappeared. From the not unfrequent occurrence of the vascular 
woody cylinders deprived of bark, I suspect that they have not been so often involved 
* “On the Structure and Affinities of Sigillaria, Catamites, and _ Calamodendron, by J. W. Dawson, LL.D., 
F.R.S., &c.j Principal of M‘Gill University,” Quarterly Journal of the Geological Society, London, May 1871. 
