546 MR ROBERT KIDSTON ON 
character has been observed.* It is therefore seen that the isolated strand type of | 
primary wood, and that even in the same specimen, passes into the continuous type, and — 
that there is between the two forms an unbroken chain which connects them together.t 
If the structure of Sigillaria elongata or Sigillaria elegans be compared with the 
structure of the large specimen of Lepidophloios described by SEwarp and Hitz, and 
which they believe to be the Lepidophlowos Harcourti, Witham, sp., or with any of the 
Lepidodendroid stems whose primary wood is provided with a corona, the great 
similarity in structure is very apparent. The corona on the stems of Lepidophlovs, 
though less prominent than in Sigilaria, is most distinctly present, and is also formed 
by the protoxylem elements. The leaf traces are also given off from the dividing bays, 
and the difference between the corona of many of the Lepidodendrex and the Sigillariz 
is only one of degree. If a series be arranged, beginning with Lepidophloios Har- 
courtt, followed by Sigillaria elongata, and concluding with Sigillaria elegans, it will 
be seen that by a gradual increase of the size of the teeth of the corona you pass 
insensibly from the Lepidophlovos structure to that of the Sigillarzz which possess the 
continuous ring of primary wood. The distinction at one time supposed to exist between 
the Srgillariz and the Lepidodendrex, of the former possessing secondary wood in con- 
nection with the leaf trace, is now found not to hold, for a considerable development of 
secondary wood takes place in the leaf traces of Lepidophilowos.§ On the other hand, no 
development of secondary wood on the leaf traces of Sigillaria elongata or Sigillaria 
elegans has yet been observed. 
Whether we are justified in classing all of the Lepidodendrex with a corona on the 
primary xylem with Lepidophloios may be open to question, though it is certain that 
some Lepidophioios had primary wood so formed ; and though on some stems of Lepido- 
dendron the primary wood has an even contour, still in other Lepidopendra and in Both- | 
rodendron || the primary wood has a slightly undulating outline, so that in the Car- 
boniferous Lycopodiacee there is a continuous chain of structure variation in the 
arrangement of the protoxylem elements which binds closely together all the genera of 
the Carboniferous Arborescent Lycopods. Between no two genera is there any out- 
standing character in the structure of the vascular cylinder which sharply separates 
them from each other. It seems, therefore, highly probable, as suggested by ZErLume, 
that the Carboniferous Arborescent Lycopods have descended from a common stock.1 
In their fructification and certain other points, however, these ancient lycopods differed 
from each other in several important characters. 
I am inclined to regard the Arborescent Lycopods as a group which has left no 
* Renavtt, Bassin howiller et permien d’Autun et @Epinac, Flore fossile, Deux. part, p. 238, 1896. 
+It might be mentioned that Renavunr has described a Lepidodendron (L. Jutiert) in which the vascular system is 
formed of a circle of separate bundles. “Structure comparée de quelques tiges de la flore carbonifere ” (in Now. 
Archives du Mus., ii., 2° sér., 1879, p. 258 ; also Runavtt, Cours d. bot. fos., vol. ii. p. 28, 1882. 
{ Trans. Roy. Soc. Edin., vol. xxxix. p. 907, pls. i—iv., 1900. § Sewarp and Hitt, l.c., p. 914. 
|| From the discovery by Mr Jamms Lomax of a specimen showing the outer surface of the bark, it has been shown 
that the Lepidodendron mundum, Williamson, is a Bothrodendron. 
{ ZuitLER, Eléments paleobotanique, p. 178, 1900. 
