548 MR ROBERT KIDSTON ON 
The lowest horizon from which I have seen the typical Clathrarian Sigillarie is 
the Lower Coal Measures, but from this horizon I have only seen a single example. 
The non-ribbed Sigillarie of the Ulodendron-Clathrarian group (Szgillaria disco- — 
phora, Konig, sp. = Ulodendron minus, L. & H., and Sigillaria Taylori, Carr., sp.) 
extend into both divisions of the Lower Carboniferous, but the ribbed Sigillaria, — 
although they occur in the Lower Carboniferous, do not, as far as I know, extend to 
the base of the Carboniferous Limestone Series. 
If I am correct in believing that the stem whose structure has been described by © 
Professor WeIss as “a Biseriate Halonial Branch of Lepidophloios fuliginosus” is the 
Sigillaria discophora, Konig, sp., with which Sigidllaria Taylori, Carr., sp., is very closely 
related, then the probability is that Srgillaria Taylor also possessed primary wood of 
the continuous ring type, and the same may be presumed for the two-ribbed Sigillarie 
from the Carboniferous Limestone Series. 
If it is permissible to assume these probabilities—and the assumption is not without 
some support from the known structure of the Middle and Lower Coal Measure species 
of Sigillaria—then it is probable that the continuous ring of primary xylem is the older 
type of Sigillarian stem structure, and that the circle of isolated strands which form the 
primary xylem of the Clathrarian Sigillarize of the higher geological horizons has 
originated by a splitting up of the continuous ring type of bundle; and, as already 
mentioned, even in the few Clathrarian Sigillarize from the higher horizon of which the 
structure is known, the actual transition from the one type to the other can be observed. 
The Lepidodendra form, however, an older genus than Szgillaria, and extend to 
the base of the Carboniferous Formation. In beds not far above the base and low down 
in the Calciferous Sandstone Series specimens of Lepidodendron showing structure have 
been found; and of two of these occurring in the same bed, one species shows the ~ 
continuous ring of primary wood, while the other possesses a solid cylinder of primary 
wood without any trace of pith ; and although there occur here the two types of primary 
wood, side by side, still the solid cylinder type seems to be more common in the lower 
than in the upper horizons of Carboniferous Rocks, and the sequence of changes in the 
development of the primary xylem of the paleeozoic Arborescent Lycopods seems to 
point to the sold vascular cylinder as the oldest type, from which has been derived the 
medullate cylinder with a continuous ring of primary wood, and this continuous ring of 
primary wood has, in turn, broken up to form the zsolated strands of primary wood 
found in the Clathrarian Sigillarizx.* 
I wish to express my thanks to Mr D. T. Gwynnz-Vaucuan, Glasgow University, 
for much kind criticism and advice while preparing this paper. 
* See note on p. 546 (Lepidodendron Jutiert, Renault). 
