866 PROFESSOR W. C. WILLIAMSON AND DR. D. H. SCOTT ON THE 
photograph 2; Plate 77, fig. 3, &c.) may, however, characterize certain branches, or 
even, perhaps, certain species. 
(b) In the secondary growth of the vascular cylinder. In every stem of the types 
examined by us secondary thickening took place; only the very smallest twigs are 
destitute of it. So far as the evidence before us shows, there is not the slightest 
reason to believe in the existence of any Calamite which did not form a secondary 
zone of wood. Its formation has been observed at every stage. The radial arrange- 
ment of the elements is exceptionally clear as compared with most recent plants 
which have secondary growth. The phloém and cambium are rarely preserved, but, 
as we shall see, they can be demonstrated in favourable cases. 
(c) In the secondary growth of the cortex. It attained, in some old stems, a 
thickness even greater than that of the wood, and its increase was accompanied by an 
extensive development of periderm. To this subject we shall return. 
There can be no doubt as to the specimens in question forming a perfectly natural 
group. With the exception of the Calamopitus type, and of Calamodendron, which 
latter we have not dealt with in this paper, all the forms examined might even have 
belonged to a single species, though it is much more probable that several allied 
species are included. 
There is, further, no doubt that the specimens showing structure, with which we 
are more particularly concerned at present, belong to the same plants as the macro- 
scopic specimens long known under the name of Calanutes. The proof that the 
common form of preservation of these fossils represents merely the cast of the hollow 
pith, has been sufficiently enforced in previous memoirs.* Specimens, such as one in 
the WILLIAMSON collection, in which a cast of the pith is still partly enclosed by the 
well-preserved cylinder of wood, show quite clearly that the furrows on the surface of 
the cast correspond to the inner edges of the vascular bundles, while the ridges of the 
cast fit into the spaces left by the softer tissue of the broad medullary rays. Medullary 
casts from the hase of branches, of which several are figured, agree perfectly with the 
form of the pith, as shown in specimens of the same parts, with structure perfectly 
preserved (see Plate 79, fig. 20; Plate 80, fig. 22; Plate 73, photographs 7, 8,and 9). In 
cases of doubt the medullary casts can be identified by their having a constriction at 
each node, while the opposite condition is conspicuous where the wood and cortex are 
present. (Compare the figures of casts in Plate 86 with the photograph 4, Plate 72 ; 
and with “ Organization,” Part XIV., figs. 5 and 6, ‘ Phil. Trans.,’ 1888, vol. 179, B). 
The cortex presents no superficial ridges and furrows. 
We may assume, then, that we are dealing with the histological structure of the 
same plants which have so long been known under the name of Calamites, of which 
the medullary casts are the most familiar form of preservation. The commonest type 
* See “ Organization,” Part I., ‘Phil. Trans.,’ 1871, and Part IX., ‘ Phil. Trans.,’ 1878, Plate 2. See 
also Srur, “ Zur Morphologie der Calamarien,” ‘Sitzber. d. k, Akad. der Wiss. z. Wien., Math.-naturwiss, 
Classe,’ Bd. 83, Abth. 1, Heft. V., 1881. Soims, ‘ Fossil Botany,’ p. 301. 
