have successively occupied the surface of the Earth. 193 



selves chiefly in forms unusual and sometimes so anomalous, that 

 it is a matter of hesitation whether to place them in this division 

 or in the preceding ; as for example the Asterophyllitese. Sub- 

 sequently, on the contrary, these anomalous and ambiguous 

 forms, the classification of which is often obscure, disappeared ; 

 the acrogenous Cryptogams and the gymnospermous Dicoty- 

 ledons become evidently referable to families still existing, from 

 which they only differ as generic forms ; the Ferns and Equise- 

 tacese which represent the Acrogens are less numerous; the 

 Coniferse and Cycadacese almost equal them in number, and or- 

 dinarily surpass them in frequency, more particularly in the 

 second period. By their abundance and their dimensions they 

 become the essential characteristic of all these formations. Finally, 

 the angiospermous Dicotyledons are still wholly wanting, and 

 the Monocotyledons are veiy few in number. 



This kingdom of the gymnospermous Dicotyledons is divisible 

 into two periods : the first, in which the Coniferae predominate 

 and the Cycadacese scarcely appear ; the second, when the latter 

 family becomes predominant by the number of species, their fre- 

 quency, and the variety of the generic forms. The second period 

 is divisible into several epochs having peculiar characters. 



3. Vosgesian Period. 



This period, which does not appear to have had long duration, 

 and which only comprises the gres bigarre, properly so called, 

 presents the following characters : 1. The existence of a tolerable 

 number of Ferns of forms frequently anomalous, manifestly con- 

 stituting genera now destroyed and occurring no more even in 

 the most recent formations ; such as the species of Anomopteris 

 and Ci'einatopteris ; the stems of arborescent Ferns are more fre- 

 quent than in the Jurassic period ; true Equiseta are very rare ; 

 the Calamites, or perhaps rather the Calamodendra, are abund- 

 ant. 2. The Gymnosperms are represented by the two Coniferous 

 genera Voltzia and Haidingeria, the species and specimens of 

 which are very numerous. On the other hand, the Cycadacese 

 are veiy rare ; M. Schimper only cites two species founded on 

 two unique specimens, very imperfectly preserved, and the deter- 

 mination of which may even be doubtful. 



This consideration appears to me completely to separate, in a 

 botanical point of view, the period of the gres bigarre from the 

 epoch of the Keuper, although both are placed by geologists in 

 the triassic formation ; for in the Keuper the Cycadacese become 

 very abundant, perfectly characterized, and frequently analogous 

 to those of the Jurassic period ; while, on the other hand, the 

 Coniferse of the gres bigarre are absent in the latter formation. 



