ANALYSIS OF THE DAKOTA GROUP FLORA. 227 



counts little by its ahsoiice, ;is the leaves of related species ot" tlie Lauriuea?., 

 iu(litfer(Mitly retV^rred to Laurus or Persea; Aristolocliia,, oue species of 

 which is described by Heer from a single leaf in the I'JiyllUes Cretacees dii 

 Nebraska; Viburnum, Acer, Elseodendron and Crataegus, which may jjcloiig 

 to moi'e recent types, as they have representatives in the Cretaceous of 

 Patoot; Hamamelites, Paliurus and Zizyphus, of which the few species ob- 

 served belong to the Dakota Group, and are also rejjresented at Patoot. 

 The genus Protophyllum, which is remarkably abundant in the Dakota 

 Group, takes there the place of Credneria, whose remains are so character- 

 istic of the Cenomanian of Germany. From the schists of Atane, Heer has 

 described as Credneria a single poorly preserved leaf, possibly referable to 

 a peculiar genus derived from Protojshyllum, but he has not descril)e(l 

 species of Pi'otophyllum. Of the number of species identilied in both the 

 flora of the schists of Atane and that of the Dakota Group, we have in the 

 ferns one only, Asplen'mm Dicksoruaimm Heer, which was first described 

 from tlie Lower Cretaceous of Kome. In the Cycads nearly the same 

 number of species are recorded in each flora, but none are identical. Of 

 the Conifers, four are identical,. and in the dicotyledonous series there are 

 identified two species of Myrica, one of Quercus, three of Populus, one of 

 Platauus, two of Ficus, three of Laurus, one of Cinnamomum, one of Sas- 

 safras, two of Diospyros, two of Andromeda, one of Cissites, three of Mag- 

 nolia, two of Liriodendi-on, one of Juglans, and four of Leguminostr, making 

 in all thii'ty-four species, or about the one-fourteenth part of the whole 

 number of species desci'ibed from the Dakota Group. '^Phe proofs of tlu^ 

 synchronism are really conclusive. For if we consider the distance Ijetween 

 the localities, which are separated l>y at least 35° of latitude, and conse- 

 (juently the probable diiference in the atmospheric circumstances, with its 

 bearing upon the vegetation, the uatm'e of the groimd, etc., the relation of 

 the floras ajipears far more distinctly marked than is general between two 

 groups of plants of the same geologic age; the Miocene, for example, 

 represented by unlike species even at localities merely a few miles distant 

 from each other. But how can we explain the predominance of the Ferns 

 and Conifers in the Flora of Atane and the scantiness of plants of this kind 

 in that of the Dakota Group! Atmos2)heric humidity governs the land 

 vegetation in its iiatxu-e and distribution. By its degree it regulates the 

 climate and the seasons. At the present e[)och this generallv known fact is 

 evidenced in the peculiar character and habitat of some groups of plants, 

 the Ferns and Conifers especially, which generally thrive upon moist 



