396 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
*025, which is its thickness on the upper side of the specimen figured, whilst on the 
lower one it contracts to -008. This inequality in the thickness of the ligneous zone on 
opposite sides of the stem is an almost invariable characteristic of this plant. In some 
specimens the zone is of such small dimensions as to be easily overlooked by observers 
ignorant of its aspect in more developed examples; where such is the case, the 
vessels are also of much smaller size than elsewhere. The largest vessels are always 
found in the thickest part of the zone : here they frequently reach a diameter of '0032 ; 
but in those parts where the zone is dwarfed their diameter does not usually exceed 
*0017. In the case of the thicker laminse, one or two of the innermost vessels are 
always smaller than the more peripheral ones. Whilst many of these vessels are 
reticulated, like those of the medullary axis, others are frequently scalariform : the 
latter is especially the case with those nearest the medullary axis. The radial section, 
Plate XXIX. fig. 33, partly traverses the outer part of the medullary axis ( b and c), and 
partly the ligneous zone ( d ). The vessels ( d ) belong to the latter ; and at j\ f we have two 
medullary rays, the cells of which are of the mural type of parenchyma, being repetitions 
of what we found in I). Oldhamium , except that the transverse diameter of each ray is 
less in the present case than in the Lancashire species, though relatively large in pro- 
portion to the thickness of the entire ligneous zone. The latter has evidently been of 
a very lax character, differing materially in this respect from that of 1). Oldhamium'. 
I have never counted more than a dozen vessels in each of its radial series. At their 
inner margins the laminae are usually connected with some one of the numerous peri- 
pheral clusters of medullary vessels ; they very rarely, if ever, commence amongst the 
medullary cells. I shall shortly call attention to some curious clusters of vessels that 
occasionally interrupt the continuity of this ligneous zone. Plate XXIX. fig. 33 a repre- 
sents a tangential section of this radiating zone. The medullary rays (f) are here seen to 
consist chiefly of a single vertical series of cells, and are further remarkable for the long 
vertical range of each ray. The figures and descriptions already given apply to the stems 
of this plant in their normal state ; but it is only in rare examples that we meet with 
them in this undisturbed condition. The physical circumstances that have attended their 
mineralization have usually compressed and displaced the tissues in so great a measure 
that it becomes almost impossible to make out the structure of such specimens. The 
vessels have lost their cylindrical character, and the entire vascular axis is distorted 
into a confused mass of cells and vessels. 
The bark of this plant is as interesting as it is remarkable ; but I have found the 
minute interpretation of its varying aspects a difficult task, owing to the effects of the 
pressure alluded to in the last paragraph. The general outline of the plant is never 
the same either in different stems, nor even, as shown on Plate XXX., in different 
sections of the same stem ; and I have found it no easy matter to determine how much 
of this irregularity was due to pressure and how much to the original peculiarities of 
the plant. The broad features of the bark are sufficiently obvious : it consists of three 
very distinct layers, the innermost two of which contain a few large, vertically disposed 
