293 
the Periderm of Fossil Lycopodiales. 
and some Stigmariae, was rarely at all constant, partly perhaps a conse- 
quence of the phellogen frequently not forming a regular and continuous 
line of cells. Reliance has not been placed on the size or flattened appear- 
ance of some of the cells, unless accompanied by less thickness of the walls, 
and where the probable position of the phellogen is indicated, this has 
in all cases been coupled with the absence of any signs of it in other parts 
of the tissue. 
The phellogen can still be distinguished in 
(a) Lepidodendron selaginoides. In this species the phellogen is 
sometimes well preserved, as at ph in Text-fig. 7, A, where the thin-walled 
dividing cells may be clearly seen. Nearly all the secondary tissue formed 
is phelloderm, there being only one or two rows of cells on the outer side 
of the meristem. In older examples a few more layers of cells may be cut 
off towards the exterior. This determination agrees with that arrived at by 
nearly all observers except Hovelacque. 1 
( b ) Sigillaria sp. ( Rhytidolepis ). The dividing cells are here found on 
the outer border of the secondary tissue, which therefore may be regarded 
as entirely phelloderm (Text-fig. 7, b). 
In the following species there are indications of the position of the 
phellogen : 
(a) The bulk of the secondary tissue appears to be phelloderm, with 
possibly one to four or five layers of cells formed on the outer side of the 
phellogen, for, although there are signs of recent divisions, it cannot be said 
exactly which are the meristematic cells (Text-fig. 8). 
L epidodendron in termediu in 
„ obovatum 
Lepidodendron sp. 
Lepidophloios (type of Lepidodendron fidiginosum ) 
Lepidophloios sp. 
Sigillaria scutellata 
Sigillaria elegans 
Sigillaria sp. 
Arber and Thomas came to the same conclusion about the position of 
the phellogen in Sigillaria scutellata , 2 and this is now further supported by 
the specimen shown in Text-fig. 7, B, which also belongs to the Rhytidolepis 
section. It should be noted that in all the ribbed Sigillariae the periderm 
is split off internally. 
In the Lepidophloios from the Williamson Collection (C. N. 1955) 
shown in Text-fig. 8, a, this determination does not agree with that of 
Seward, who states for a similar specimen, consisting of leaf-bases and 
1 Hovelacque ( 15 ), pp. 29, 158. 
2 Arber and Thomas ( 1 ), p. 141. 
