OP THE POSSIL PLANTS OP THE COAL-MEASUEES.

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we have already seen, this is the condition existing in the small Coalbrookdale speci-men ; but it is not the usual state of the pith in larger Dadoxylons , in which the Stern-bergian laminae are only enclosed within a very thin cylinder of undivided medullaryparenchyma.

Fig. 42 is a transverse section of a large branch of the natural size. The medullahas been three quarters of an inch in diameter, and the entire stem, exclusive of itsbark, was, when perfect, 2-g inches in diameter, the ligneous cylinder having a maximumthickness of nearly an inch. Though the woody wedges composing this cylinder havebeen disturbed by mineralization, it is obvious that the medullary extremities of thelarger ones projected into the medulla, as is so commonly the case in the branches ofliving Conifers (e. g. Araucaria imbricatci), giving to the transverse section of the pith astellate outline, large pointed rays of the latter being prolonged outwards between theconvex extremities of the wedges. This specimen, for which I am indebted to JohnAitken, Esq., of Bacup , came from the Halifax Bullion or Ganister bed at South Owram.The matrix is full of small Goniatites. The longitudinal section (fig. 43) of the abovebranch displays some peculiarities. Thus the medullary layer lining the vascular cylinderis very thin, soon breaking up into thin disks, hut which are very regularly thickened attheir circumferential portions; hence, in the vertical sections, each disk appears as a pillarstanding upon a rapidly swelling base (fig. 47, a), which latter rests upon the interior ofthe woody cylinder ( b ). Then the laminae of the several disks, instead of extendingstraight across the medullary cavity, subdivide into two, one half joining the corre-sponding half of the lamina above, and the other half being united in a similar mannerto the lamina below. Thus there is formed a central pile of lenticular cavities, a , whichalternate with a more marginal series of lenticular rings, a!, a!.

Fig. 44 is a transverse section of a small branch from one of the bullions above the two-foot coal at Oldham ; and fig. 45 is a vertical section of the same specimen. Its diameteris half an inch, of which the medulla occupies nearly T9. This latter tissue not onlydivides into the usual disks, a , but each one of these again exhibits the strongest tendencyto split up into numerous yet thinner layers, each one of which consists apparently of asingle horizontal layer of cells*. It is obvious that in this instance the medullary cellshave arranged themselves in successive vertical series of horizontal layers, and thus pre-disposed the tissue to break up into thin laminse along the lines intervening between

* In pointing out the fact that the Balsam fir {Abies balsamea ) has a discoid pith, Dr. Dawson says :■—“ This modern Sternbergia is not produced by the mere breaking of the cellular tissue transversely by elonga-tion of the fibre, but, as I pointed out many years ago in the case of the Coal-formation Sternbergice (‘ Cana-dian Naturalist and Geologist,’ 1857), is a true organic partitioning of the pith by diaphragms of denser cellsopposite the nodes, as in Cecropia peltata and some species of Ficus. The transverse diaphragms are com-posed of denser cells flattened horizontally, and they are, as in Sternbergia, accompanied by constrictions of themedullary cylinder.”— Nature , May 15, 1873.

In my Sternbergia described above I find no evidence of two kinds of cells ; but there is an unmistakablehorizontal stratification of them which obviously approximates closely to Dr. Dawson’s description.

MDCCCLXXVII. 2 K