the Earth at different Epochs^ ^c. 357 



nera, and even tlic families, are different. Thus, the Cvcadea* 

 appear, ft)r the first time, at this epoch, and continue, with some 

 modifications in their forms, until the end of this period, of 

 which they constitute the essential character. This family, and 

 that of the Coniferae, which we consider as composing tc^ether 

 a particular class, did not then, as now, form scarcely a hun- 

 dredth part of the existing vegetables, but, on the contrary, com- 

 posed of themselves alone the half of the flora of this period of 

 vegetation, and of the thirty-five species of this class that have 

 hitherto been observed in these formations, twenty-nine arc Cy- 

 cadea^. This family, therefore, was more numerous at an 

 epoch, when the flora of the earth appears so poor, and so little 

 diversified, compared with its present state, when more than 

 fifty thousand living plants are known. 



The rest of the flora of this third period of vegetation is al- 

 most entirely formed by the vascular cryptogamic plants, such 

 as the Ferns, the Equisetaceae, and the Lycopodiaceae. Thus, the 

 essential character of the vegetation of this period, consists in 

 the great numerical predominance of the Cycadeae, and in the 

 nearly equal division of ail the vegetables into two classes, that 

 of the vascular cryptogamic plants, and that of the gymnosper- 

 mous phanerogamic plants. 



There exist but very rare indications of Monocotyledones^ 

 and nothing yet announces the existence of Dicotyledones. 



The presence of dicotyledonous vegetables, and the numeri- 

 cal predominance which they acquire over the other vegetables, 

 the moment they make their appearance, characterize the fourth 

 period of vegetation, that which corresponds to the formations 

 analogous to those of the Paris basin, and which are designated 

 by the names of Upper Sedimentary, or Tertiary Formations. • 



At this epoch, the proportion of the different classes of plants 

 appears to have been nearly the same as upon the present sur- 

 face of our globe. The Dicotyledones are at least four or five 

 times more numerous than the Monocotyledones. As to the 

 other classes, the peculiar circumstances in which these deposits 

 have been formed, appear to have diminished their number. 

 There are only found some traces of Ferns, Equiseta, and 

 Mosses, and the agamic plants are only represented by various 

 species of marine vegetables. The plants of this period gene- 



