On account of these doubts, M. Brongniart indicates these three floras separately; 1st, The Flora of the bituminous slates of Thuringia, composed of algæ, ferns, and coniferæ; 2d, The Flora of the Permian sandstones of Russia, which comprehends ferns, equisetaceæ, lycopodiaceæ, and nœggerathiæ; 3d, The Flora of the slates of Lodève, which is composed of ferns, asterophylliteæ, and coniferæ.
"We perceive that there are great specific differences between the plants of these localities, and that hitherto no species common to them has been found. Must we ascribe these differences to the influence of the great diversity of geographical position, or is there, besides, a difference in the period of their origin among these formations? The only character which tends to bring these two latter Floras near each other, is the relation which both of them bear to the coal-formations, of which they seem to be a kind of extract, reminding us more especially of the most recent beds.
"With regard to the plants of the bituminous slates of the Mansfeld district, they are so few in number, and appear to have been deposited in conditions so different, that we can with difficulty compare them with the two other Floras. Yet the species of Sphenopteris are extremely like each other in the three formations, and an exact comparison would perhaps establish the identity of many of them. The Pecopteris crenulata of Ilmenau, is only perhaps an imperfect state of the Pecopteris abbreviata of Lodève; lastly, the Callipteris of the Permian formation of Lodève have a very close connection between themselves and the Callipteris of the coal-formation.
"We may add, with regard to the bituminous slates of Thuringia, that many of these fossils appear to be marine plants, whose numbers would become much more considerable if we did not suppress all the imperfect impressions which have been described as such, and which are nothing more than fragments of ferns or altered coniferæ.
"II. Reign of the Gymnosperms.—During the preceding periods, and particularly during the Carboniferous period, the Acrogenous cryptogams predominated, and the Gymnospermous dicotyledons, less numerous, shewed themselves in unusual forms, and sometimes so anomalous that we are in doubt whether to place them in this or the preceding department; such are the Asterophylliteæ. At a later period, on the contrary, these anomalous and ambiguous forms, whose classification is often obscure, disappear; Acrogenous cryptogams and Gymnospermous dicotyledons evidently enter into families still existing, differing from them only in generic forms; the Ferns and Equisetaceæ, which represent the acrogens, are less numerous; the Coniferæ and Cycadeæ almost equal them in number, and usually exceed them in frequency, especially in the second period; by their abundance and size they afford the essential character of all these formations; lastly, the Angiospermous dicotyledons are wholly wanting, and the monocotyledons are in very small numbers.
"This reign of the Gymnospermous dicotyledons is divided into two periods; the first, in which the Coniferæ predominate, and in which the Cycadeæ scarcely appear; the second, in which this family becomes predominating in the number of species, in frequency and variety of generic forms. The latter may be divided into many epochs, each presenting peculiar characters.
"III. Vosgian Period.—This period, which does not appear to have been of long duration, and comprehends only the variegated sandstone properly so called, presents the following characters; 1st, The existence of ferns, pretty numerous, of forms very often anomalous, evidently constituting genera now extinct, and which are not found even in the most recent formations; such are the Anomopteris and the Crematopteris. Stems of arborescent ferns are more frequent than during the Jurassic period; true Equisetums are very rare; the Calamites, or rather perhaps the Calamodendrons, are abundant. 2d, The Gymnosperms are represented by two genera of Coniferæ, Voltzia and Haidingeria, of which the species and specimens are very numerous. The Cycadeæ, on the contrary, are very rare. M. Schimper mentions only two species founded on two unique specimens of a very imperfect character, and the determination of which may be considered doubtful.
"This consideration appears to me to separate completely, in a botanical point of view, the period of the variegated sandstone from that of the Keuper, although both are placed by geologists in the trias-formation. For the Cycadeæ become very abundant in the Keuper, are perfectly characterised, and often analogous to those of the Jurassic period; while the Coniferæ of the variegated sandstone are, on the contrary, wanting in this formation.
"IV. Jurassic Period.—This period is one of the most extensive by the formations which it comprehends, and the variety of different special epochs of vegetation which it embraces; although we cannot refuse to comprehend, under a common title, epochs during which very analogous forms have succeeded each other. It thus comprehends from the Keuper inclusively, up to the Wealdean formations. In fact, we find the Pterophyllum of the Keuper appear anew, with slight specific differences in the Wealdean formations. The equisetites of the Keuper extend to the mean oolithic formation; the baiera of the Lias likewise recurs in the Wealdean beds of the north of Germany; the Sagenopteris and the Camptopteris likewise appear in the Keuper, Lias, and Oolite.
"Yet these common characters, which indicate a great analogy between the Floras of each of these epochs of formation, do not prevent each of them having characters of its own, and often an assemblage of species, almost all peculiar to each particular epoch. We ought, therefore, to distinguish here those various subdivisions, the number of which will perhaps be afterwards multiplied, when we become better acquainted with the vegetables of each of the stages of the Jurassic formations.