684 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
measured from side to side ; when unrolled they would be much more. The other beaded 
structures (tig. 17) were fragments of various lengths, from a quarter of an inch down- 
wards, often arranged in pairs, or even in more or less parallel sets of four, as in the 
upper part of tig. 17, l'. By making sections of these two forms at right angles to each 
other, as in tig. 17, I succeeded in obtaining proofs that the revolute objects (tig. 15) 
were merely transverse sections of structures, of which the beaded lines (tig. 17, l') were 
longitudinal ones. After months of research, involving labour apparently very dispro- 
portionate to the end attained, I obtained further proof that these were sections of 
leaflets of Pecopteris , a nearly perfect one of which is represented in Plate LIII. 
fig. 19. It appears that even in their mature form these leaves were apt to assume a 
remarkably revolute form. Hence the great number of sections obtained like Plate 
LIL fig. 15. Specimens like figures 17 & 18 of Plate LIII. showed that the white 
head-like spaces represented the intervals between the nervures, and the dark dividing 
lines were the nervures themselves. In the double-beaded line at the lower part of 
fig. 17 the section has only passed through one lateral half of the revolute leaf; in the 
upper part of the same figure, owing to the twisted position of the leaf, it has passed 
similarly through both halves, hence the four beaded lines l , l The mid rib of the 
leaf appears at the points indicated by l", l ", l". In the central part of the figure the 
dark lines of the nervures are seen proceeding upwards and outwards from the mid rib 
towards the left-hand margin of the leaf, the section being here made in the plane of 
that half of the leaf without passing entirely through it as at l. The right-hand margin 
of fig. 18 is in the same condition as is the centre of fig. 17. 
The exceeding abundance with which fragments like those just described are asso- 
ciated with the specimens represented by all the figures from 1 to 14, early suggested 
to me the possibility that they all belonged to the same plant. On this supposition 
figs. 13 & 14 would belong to young and half-developed fronds, whilst figs. 15, 17, 18, 
& 19 were parts of matured leaflets.- Plate LIL fig. 16 represents one of several 
specimens which seemed to sustain this idea. It looks like a section of a revolute leaf- 
let resembling fig. 15, yet it undoubtedly belongs to the leaflet series of figs. 13 & 14 ; 
as is the case with all the leaflets of the latter specimens, its obtuse margins indicate 
a more succulent state than do the sharply defined incurved edges of Plate LII. fig. 15. 
If fig. 16 is really a transverse section of one leaflet, and not a combination of two 
leaflets intersected longitudinally , the identity of the two forms would be almost cer- 
tain ; but I have not been able to link the two forms in a perfectly satisfactory manner. 
There is also the further difficulty, that the objects represented in figs. 15, 17, 18, & 19 
are leaflets of a Pecopteris of the type of P. Serlii, none of the pinnules of fronds of 
which type are constructed on the plan of figs. 13 & 14. I am therefore now inclined 
to conclude that Pachiopteris aspera (as I propose provisionally to designate the petioles 
just described) is a Sphenopteris. It is an important fact in reference to this question, 
that we have in the Coal-measures a Sphenopteris with a tuberculated rachis, and which 
has a wide geographical range. This is the S. Hoeninghausi already referred to. The 
