Relation of Pteridosperm Anatomy to that of Cycads. 4 7 
peculiarity of structure foreshadowed in some specimens of 
Lyginopteris oldhamia —the formation of one or even two inverted 
zones of wood and bast at the margin of the pith ; and it is in this 
fact, coupled with that of the Cycadean nature of the secondary 
wood, that the interest of the fossil, from the present point of view, 
lies. 
Cycadoxylon Fremyi (10; 14, p.486) shows the same type of 
structure as Cycadoxylon robnstum. There are two or more 
interrupted inverted zones of xylem and phloem, separated from 
the normal zone by parenchyma. Although there is still, according 
to Seward (15, p. 79), a trace of the mesarch structure of the xylem 
strands in the stem, it is clearly much reduced, so that the wood is 
very Cycad-like. 
Ptychoxylon Levyi (Fig. 5) is another type of the Cycadoxylese 
in which extra zones of vascular structures occur (10; 14, p. 487). 
An outer more or less continuous cylinder of wood and bast 
surrounds the large pith, which contains several secondary vascular 
bands. Leaf- and branch-traces break the continuity of the central 
Fig. 5. —Ptychoxylon Levyi. Transverse section of a decorticated stem. 
x,ph, normal xylem and phloem; x', pli', first inverted medullary bands of 
xylem and phloem, continuous at the leaf-gaps with normal zone; x", ph", 
second inverted zone ; by, stele of a branch. (Adapted from Renault, 1896j, 
