"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. "_Keupric Epoch._--M. Brongniart then gives an enumeration of the vegetables of the Keupric epoch, which, in regard to the Amphigenous cryptogams, consist of Algæ; in regard to the Acrogenous cryptogams, of Ferns and Equisetaceæ; in the case of the Gymnospermous dicotyledons of Cycadeæ and Coniferæ; lastly, of two doubtful monocotyledons (_Palæoxyris_ and _Preisleria_.) "On comparing this Flora with that of the variegated sandstone of the Vosges, and with that of the Lias, we perceive that it has nothing in common with the first except the palæoxyris, which appears very nearly related to that of the variegated sandstone; on the contrary, it resembles the Flora of the Lias or Oolite in the ferns, many of which are specifically identical, or nearly allied in the _Nilsonia_ and _Pterophyllum_, which are likewise either identical, or very nearly connected specifically with the Lias. "_Lias Epoch._--The Liasic epoch furnishes Amphigenous cryptogams, consisting of Algæ, mushrooms, and lichens; Acrogenous cryptogams, such as Ferns, Marsileaceæ, Lycopodiaceæ, and Equisetaceæ; Gymnospermous dicotyledons, represented by the Cycadeæ and Coniferæ; finally, doubtful monocotyledons, consisting of _Proacites_ and _Cyperites_.