Palæoxylon (coniferous wood of the Coal Measures).—The existence of coniferous trees in the Carboniferous flora, and the fact that their trunks and branches had contributed to the formation of coal, was first discovered and clearly demonstrated by Mr. Witham in the work to which reference has been made. Figures of the peculiar structure observable in thin slices of coal, are given in Obs. Foss. Veg. pl. iii. iv. v. This carbonized wood resembles that of the Araucariæ in the multiple series of ducts, and their alternate arrangement; but the presence of thick compound medullary rays in these stems,—a character unknown in any living conifers,—led M. Brongniart to place them in a separate genus—Palæoxylon (ancient wood); characterized by the presence of medullary rays formed of numerous layers of cells, which are not arranged in superimposed series, and that present a lanceolate or oval form, in a section perpendicular to their direction.[137]
[137] The Pinites Withami, and P. medullaris, of Lindley and Hutton, figured in Mr. Witham's work, belong to this genus. It may interest the reader to know that slices of these woods prepared for the microscope by Mr. Nicol, (presented to me by the late Dr. Henry, of Manchester,) not only expose the vegetable organization in an admirable manner, but also form beautiful objects for the exhibition of polarization.
Peuce.—Another species of coniferous wood from the coal is thus named; it differs from the former in the medullary rays being composed but of one layer of superposed cells.
Araucarites (Dadoxylon of Endlechen).—This term is employed to designate the fossil wood whose structure is apparently identical with that of the living species of Araucariæ, having the same kind of medullary rays, and the ligneous fibres studded with discs or areolæ, which are polygonal, often hexagonal, and disposed in several alternating series. This wood is common in the Lias, Oolite, Wealden, and Chalk.
Drifted fragments of coniferous wood of this type occur in the Stonesfield slate, associated with leaves and fruits of cycadeæ, and with marine shells, bones of reptiles, fishes, and mammalia; at Scarborough, with the ferns and zamiæ previously described; at Swindon, in the Portland oolite, with belemnites, ammonites, trigoniæ, &c.
STERNBERGIA.
Sternbergia.—To the Araucarian tribe, according to the recent investigations of Professor Williamson, must be referred certain fossil stems found in the coal-measures, and named Sternbergiæ.[138] These are long solid cylindrical casts of sandstone or clay, with annular constrictions, which are generally invested with a thin film of carbonaceous matter; when this crust is removed the surface is found to be marked with longitudinal ridges. These fossils were once supposed to be the stems of plants allied to Yucca or Dracæna; but, as was first shown by Mr. Dawson and Mr. Dawes,[139] they are merely sandstone casts of the medullary axis or cylinder of an extinct genus of coniferæ, allied to the Araucariæ: a specimen in which the cast was surrounded by a thick ligneous cylinder, heaving enabled that acute observer to detect the structure of the original.[140] The Sternbergiæ are sandstone casts of central cavities existing within the true pith; which cavities, under some favourable conditions, were filled with inorganic materials. Mr. Williamson is inclined to believe that all the coniferous wood from the coal-measures, belonging to the genus Dadoxylon, is referable to the trees of whose piths the Sternbergiæ approximatæ are internal casts; and that some of the foliaceous appendages of these trees have been confounded with Lepidodendra.[141]
[138] See Pictorial Atlas, pl. xviii. p. 53.
[139] On the Coal formation of Nova Scotia, Geol. Proc. 1846.