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. [140] See Prof. Williamson's Memoir on Sternbergia, Manchester Philos. Trans. 1851. [141] Ibid. p. 355. Petrified Forests of Conifers.--The most remarkable assemblage of fossil conifers is that presented in the well-known quarries in the Island of Portland, to which allusion was made when describing the Mantelliæ obtained from that locality (_ante_, p. 157). Referring to _Wond._ p. 385,[142] for an account of the geological circumstances under which the phenomena occur, it will suffice to state that a forest of pines appears to have been submerged, and the trunks to have become petrified, whilst standing erect on the spot where they grew; the Cycads still shoot up as it were between the stems, and the roots of the trees, though changed into flint, extend into the bed of mould whence they originally derived support, and which is so little altered in appearance, as to be called the _Dirt-bed_, by the quarrymen; thus realizing the fable of the petrified city in Arabian story, whose inhabitants were turned into stone, in the varied attitudes of life. [142] Geol. I. of Wight, p. 394. Petrifactions, p. 56. No foliage has been observed in connexion with these trees; not a leaf has been found in the rocks: a cone, nearly related to the fruit of Araucaria excelsa, was discovered in the Dirt-bed.