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ON THE STRUCTURE AND AFFINITIES OF LEPIDODEN
DRON AND CALAMITES.
(Plates LV., LVI.) The imperfect knowledge we have of fossil plants is the result of the fragmentary condition in which they occur. The deciduous leaves, ripe fruits, or broken branches that fell into streams, and were carried to sea or lake, had so many dangers to encounter, that only a very few of them ever reached the usual deposit where they would be preserved, and these few in such a decayed and fragmentary condition that it is often impossible to do more than make the most vague guesses at the nature of the vegetation to which they belonged. The occurrence of vegetable remains on the site where they grew, is extremely rare in all the formations which form the crust of the earth, except in the coal-measures. The plants of this period might therefore be expected to be well known, especially as the beds containing vegetable remains, of carboniferous age, have been more exposed, because of their economic value, than those of any, or indeed of all the other formations put together. The peculiar nature of the vegetation, and perhaps the extreme humidity of the atmosphere, and the swampy localities in which the plants grew, have made the superabundant mass of vegetable remains as great a mystery as the scanty and fragmentary fossil plants of other periods. Except in the thin films of charcoal which occur in most coals, traces of structure are scarcely to be found in the coal itself, so thoroughly has the vegetable matter been converted into amorphous pulp before mineralization took place, or so completely has it been metamorphosed subsequent to deposition. The plants themselves have all been so brittle, that when portions are preserved, as they are in iinmense quantity, especially in the roof shales, they are so fragmentary, that it is difficult to determine the various portions that belong to the same plant. The root is rarely connected with the stem, the stem with the branches, or the branches with the leaves or the fruit. As a result, all these parts have been often referred to different genera, and have received different names. With additional observations, the means are, however, occasionally turning up, which enable us to reduce some of these genera, the
VOL. IV. (NOVEMBER 1, 1866.]
establishment of which was absolutely necessary in the earlier days of palæontological botany. Thus, to give an example :—the trees belonging to the same set as those which were found imbedded in the sandstones at Craigleith quarries have been constituted into the genus Dadoxylon ; the pith forms the genus Sternbergia, and some fluted and constricted specimens have been referred to Calamites. The leaves were considered to be ferns, and named Cyclopteris ; and the fruit was thought to belong to a Palm, and received the name of Trigonocarpon. We have not seen evidence sufficient to convince us that all these are correctly referred to the same plant; but this is the opinion of some distinguished palæontologists, and it serves as a good illustration of the present satisfactory tendency of palæontological botany.
A similar multiplication of generic names encumbers the synonymy of the two genera Lepidodendron and Calamites.
Lepidodendron was a branching tree of considerable size. It is separated from the other genera of coal plants by the form and arrangement of the leaf-scars upon its stem. More than forty species have been recorded; but as the scars present different appearances on different portions of the same plant, no doubt more species have been established than the materials fairly warrant. But that they were numerous in species, and very numerous in individuals, any one who has even cursorily examined a coal-pit, or the fossils in any public museum, must be convinced. They certainly contributed largely to the formation of coal.
The researches of Witham,* Lindley and Hutton,t Brongniart | and Binney, § have made us acquainted with the stem. These published * The Internal Structure of Fossil Vegetables,' 1833.
The Fossil Flora of Great Britain, 1831-1837. I 'Observations sur la Structure intérieure du Sigillaria elegans, etc.'Archives du Muséum, 1839.
§ Geological Society's Journal,' 1862, and Philosophical Transactions, 1865. Mr. Binney, in these papers, gives most careful and elaborate drawings and descriptions of some fossils in his extensive collection. He refers them to the genus Sigillaria, because of their agreement in internal structure with Brongniart's S. elegans ; but he cannot separate them by their external markings from Lepidodendron selaginoides, Lindl. and Hutt.; and as the only characters by which the two genera are distinguished are derived from the markings on the stem, we must consider Sigillaria vascularis as a true Lepidodendron. I am the more satisfied as to this, because I believe no essential difference exists, as has been hitherto maintained, between the stems of Sigillaria and Lepidodendron, or any of the other lepidodendroid plants of the coal period. I cannot enter into this question here, but I shall take an early opportunity of publishing my views, and the reasons for maintaining them.