4 THE PALEONTOLOGY OF MINNESOTA. 



[Floras younger than the Dakota. 



the number of species of Cycadece is greatly increased at some localities and most 

 probably the conifers and the ferns may be found also more abundant at some others. 

 The flora of Atane, and that of the Dakota group have a number of identical species. 



As yet, no remains of fossil plants have been described from the American 

 Cretaceous above the Dakota group. But in Greenland, at a higher stage than that 

 of Atane and in strata considered as referable to the Lower Senonian, Upper Creta- 

 ceous, the same discoverers have found a group of plants still related to the Ceno- 

 manian by some identical species, and comprising in 118 species, 1 fungus, 19 ferns, 

 1 Equisetum, 17 conifers, 5 monocotyledons and 75 dicotyledons. In this flora the 

 proportion of the dicotyledons is 63 percent., and the general character of the vege- 

 tation is evidently the same. In continuing the researches above, in the subsequent 

 formations, we would find the same kind of gradual change and the same proportion 

 in the composition of the flora. Some of the types are modified in the character of 

 the species, which either disappear or are constituted as new ; but the general 

 proportion in the constituents of the floras remains about the same. For example, 

 at the base of the Tertiary, the flora of the Laramie group, Lower Eocene in 

 character, has in its composition a proportion of 66 per cent, of dicotyledons. 

 It has, moreover, a new element in the predominance of the palms, of which 

 very few remains have been found in the Cretaceous. But above, in the Oligocene 

 where the palms have become extremely rare, the proportion of the dicotyle- 

 dons remains the same, as it is also in the Miocene, and in the flora of the 

 present epoch, being merely modified by local influences, especially by variations of 

 temperature. 



Is it then possible to explain in some way the total change noticed in the char- 

 acters of the vegetation of the earth in the middle of the Cretaceous? To show 

 the difficulty of the solution of a problem like that of the appearance of the dicoty- 

 ledons in the flora of that period, it is necessary to know something more about the 

 characters of those primitive dicotyledons, as we find them in the Dakota group. 



To admit, as do some authors, that the change has been produced by a gradual 

 modification of some types, caused by external influences, one would suppose, in 

 considering the large number of dicotyledons now known from the Middle Creta- 

 ceous, that it would be possible to find some traces of the successive degrees of mod- 

 ifications which, of course, can not have acted merely upon the leaves, or upon a 

 single kind of organ, but upon all the parts of a plant. No species of the dicotyle- 

 donous series has as yet shown any such intermediate characters indicating by its 

 inferiority a degree of transition ; and thus, of all the species found in the strata of 

 the Middle Cretaceous, it is not possible to consider any one as being of a lower 



