70 On the Structure and Affinities of [^TuS.TS?ri86? 
the oblique parallelism of the original type of structure is still dis- 
tinctly preserved. In this vessel a few of the pits are surrounded hj 
an areola (Fig. 6, h), forming a true coniferous disc. In this indi- 
vidual fibre the addition of the areola is accompanied by a corre- 
sponding addition to the central dot, giving it an irregular crucial 
form, which does not exist in the " pits " that are not so furnished ; 
demonstrating that the addition to the oblong dot converting it 
into a crucial one is external rather than an internal — such additions, 
however, are exceptional, though not rare. What causes determine 
the selection of special dots for areolation is uncertain; but we 
have here a manifest illustration of the process by which a spiral or 
scalariform vessel may be gradually transmitted into a glandular 
or discigerous one, and of the way in which the "pits" of the 
former structure regulate the positions of the discs of the latter. 
It appears clear that in my Dadoxylon (Figs. 1 and 2) we 
have the outer lenticular disc, but not the internal deposits, con- 
verting the fibre into a pitted tissue. Hence the absence of the 
central dot in each areola, affording a new instance of the differ- 
ences existing in the combinations of the elementary tissues in 
fossil as compared with recent plants, to which I have elsewhere 
called attention. In the stems of all the true conifers the discs are 
confined to the lateral surfaces of the fibres, parallel with the 
medullary rays — an arrangement to which those of Dadoxylon are 
no exception. I have observed that, in every recent conifer which 
I have studied, the " pits " opposite the discs (which are often con- 
verted into pores by the absorption of the wall of the vessel), also 
exist opposite each medullary ray, and there only, indicating some 
correspondence in the functions of the discs and the rays. 
Whilst Dadoxylon thus appears to furnish the lenticular disc 
or external element of the coniferous fibre, I think we may regard 
Dictyoxylon as supplying a modification of the internal deposits, 
and thus possessing some relationship to the conifers. That the 
general aspect of the wood has been coniferous is shown by Fig. 7, 
which represents a specimen in my cabinet from the Lancashire 
Coal-measures. It is a portion of a large stem through which 
there passes a branch or " knot ;" the undulations of the woody 
layers immediately below the branch too closely resemble those 
of a roughly-split pine-log to be overlooked. In the centre of the 
branch there appears a section of a rather large pith. 
In estimating the true significance of the reticulated deposits 
of lignine in Dictyoxylon, we must recall the varied character of 
these internal deposits in recent conifers. They assume the sim- 
plest form in the common deal and other allied plants, where a 
continuous layer of lignine lines the tube, being wanting only, as 
already pointed out, opposite the glandular discs. In addition to 
this, in the common yew a second and more internal deposit exists, 
