208 
PROFESSOK W. C. WILLIAMSON ON THE ORGANIZATION 
Corda, but they are unmistakably Lepidoclendroid in structure. The different examples 
which I have seen exhibit variations in the development of the medullary rays in the 
ligneous zone ; but I can trace no distinctive feature separating the extreme modifications ; 
consequently the one which I have figured may be accepted as a fair and well-marked 
example of its class. The characteristic feature of all these specimens is that they have 
a medullary ring of barred vessels (c) not arranged in linear or radiating order, surrounded 
by a well-developed woody zone of smaller barred vessels which are arranged in linear 
series (d), the whole having been encompassed by a parenchymatous or prosenchymatous 
bark ( g ). The centre of the axis is vacant, but whether primarily fistular, or occupied 
by some other tissue, remains to be considered. In the example figured there are as 
many as 130 vessels in each radiating linear series seen in the transverse section of the 
woody zone. On making a tangential section of a portion of the same zone (Plate XXVII. 
fig. 23), we discover that medullary rays (f) abound. Some of these, like that represented 
near the centre of fig. 23, f\ are very large, consisting of a dense mass of vascular and cel- 
lular tissue, whilst others (f) are of the smaller cellular type common to all the Lepido- 
dendra. The general arrangement of these rays is well shown in the radial longitudinal 
section (Plate XXVI. fig. 22, f), which closely resembles, so far as the rays are concerned, a 
similar section from a living Conifer. In the figures of Diploxylon given by Mr. Binney* 
the woody zone is represented as coming into contact with the medullary vascular ring 
by a series of concentric curves, the convexities of which are directed inwards. Corda 
represents the examples upon which he based his genus in the same way. But the 
specimens now under consideration do not exhibit this contour, the line of demarcation 
between the two tissues being nearly straight. The crenulated outline is also represented 
in With AM’s Anabathra ; but in this instance the irregularity is easily explained. It 
does not correspond with the line of demarcation between the two tissues. I am indebted 
to the kindness of Professor King, of Galway, for a very fine transverse section of 
Witham’s original plant, and find that the greater part of its area is broken up into a 
series of small circles, within each of which the tissues of the plant are well preserved, 
but external to which there is nothing but agatized mineral matter. The line of 
junction between the medullary vessels and the ligneous zone is similarly affected. 
Hence probably the existence in this case of the crenulated outline referred to. 
AVitiiam himself correctly refers these appearances to “ the crystallization of siliceous 
matter”f. At the same time plants of the class under consideration do exist which 
possess this crenulated inner margin of the ligneous zone. M. Brongniart has repre- 
sented one in his well-known memoir on Sigillaria elegans , and I will now call atten- 
tion to another which presents the best illustration of the structure in question that I 
have yet seen. It was obtained from the lower coal-measures near Oldham, by Mr. Nield, 
and its appearance before being cut into sections is represented in Plate XXIX. 
* Description of some Fossil Plants, &c. loc. cit. pi. 30. fig. 4. 
t The Internal Structures of Fossil Vegetables found in the Carboniferous and Oolitic Deposits of Great 
Britain, by Henry T. M. With am, of Lartington, Edinburgh, 1833, p. 40. 
