53 
Zygopteris Grayi of Williamson . 
5. ( 3 ) Manchester R. 443. 
6. ( 4 ) Kidston 308. 
7. ( 5 ) Williamson 1818 A. 
8. (6) Scott 184. 
The only doubt is as to the position of sections 1919 D and B from the 
Williamson Collection ; it is possible that they should come at the end of 
the series instead of the beginning. I have given the alternative order in 
parentheses. No question of importance is involved ; the order of 6 of the 
sections is certain, and 1919 B clearly comes next above 1919 D (see PI. II, 
Phots. 11 and 12). 
The stem is considerably smaller than in the Shore specimen, having 
a diameter of about 12 mm. as against 18 mm., the wood measuring about 
4 mm. as against 6 mm. The arms of the stele are rather shorter in pro- 
portion, and when a leaf-trace is detached only a slight protrusion is left 
(see PI. II, Phots, 11-13, and compare with PI. I, Phots, r-6). 
In the matter of preservation, the older specimen has the advantage as 
regards the phloem and the thin-walled inner cortex (see especially PI. IV, 
Fig. 12, and PI. V, Fig. 13).. The row of dark cells in the figures cited is 
probably the endodermis. The outer cortex appears to have thicker walls 
than in the Shore specimen. On its outer border a few layers of periderm 
can be recognized in places. The main features of the anatomy need not 
be further described ; they are evident from what has been said above, and 
from the figures. 1 
Williamson’s second specimen is the typical example of that form of un- 
divided leaf-trace which is approximately triangular in transverse section (PI. 
IV, Fig. 12), thus differing widely from the Shore specimen with its crescentic 
trace (PL I, Phot. 8 ; PI. Ill, Fig. 1). At one time I thought that this difference 
might be of specific value. I find, however, that among Williamson’s type- 
specimens of 1888 both forms of leaf-trace occur. Thus, an approximately 
triangular trace is shown in the stem figured by Williamson (’ 89 , PI. I, Fig. 1), 
while in the stem of which a photograph is given in the present paper 
(PI. II, Phot. 15) the trace (IJ.) is perfectly crescentic and agrees closely with 
those of the Shore plant. Hence,, whatever the variation in form may mean, 
it is impossible to base a specific distinction upon it, for no one will suppose 
that Williamson’s type-specimens, closely crowded together in the same 
block, belong to two different species. The form of the trace is, however, 
important, in so far as it has influenced the morphological interpretation of 
its nature. This question is discussed below (p. 57). 
Returning for a moment to the later Williamson specimen, it may be 
worth mentioning that it differs from the Shore plant in the form of the 
1 The anatomy of the stele is shown in my Studies in Fossil Botany, 2nd ed., Fig. 116, 
p. 309. See also Bertrand, ’ 09 , PI. XT, Fig. 78. 
