GEXEUAL CONCLUSIONS. 49 



very difficult to fix. Plaiamts has a number of species in the Cretaceous 

 one, especially related to P. occidentalis. The same type is then followed 

 by P. Hardenii of the Eocene, where other and different species are found 

 also ; by P. aceroides and P. GuUelmce of the Miocene of Carbon ; and by 

 the species of the Pliocene of California. It is the same with Juglans and 

 Carya, not positively recognized, however, in the Cretaceous, but already 

 present by different species in the Eocene of Colorado and the Mississippi, 

 and henceforth in the subsequent formations. No less than six species 

 of fossil Juglans have been described (without counting those of the Plio- 

 cene, where all the types are represented), and a fine Can/a, C. antiquorum ; 

 generally found in a profusion of specimens. Of Quercus, two of the types 

 of the present North American flora are already in the Cretaceous, — that 

 of the Q. castanea, also in the Miocene of Alaska, wherefrom Heer describes 

 a Q. pseudo-castanea, and that of Q. imbricaria. In the Eocene of Golden, 

 Q. angustiloha recalls our Q.falcala. Eighteen forms of Quercus, recognized 

 in the Ligriitic Tertiary flora, show to those of our time an analogy becom- 

 ing still more distinct by the species of the Pliocene. Castanea is Miocene, 

 or even perhaps Cretaceous, by the leaves referred to the genus Dryophyl- 

 lum of the European authors. Of Fagus, the Cretaceous leaves are not 

 distinguishable by any evident characters from those of the living P. syl- 

 vatica and P. ferruginea. Corylus is Eocene. Dr. Newberry has described 

 from the Fort Union group leaves of this genus under the specific name 

 of C. Americana and C. rostrata, while C. Macquarrii, Heer, a species inter- 

 mediate between these two, is richly represented in the Alaska Miocene 

 flora. There we have also Liquidambar, Myrica, Alnus, Betula, Carpinus, in 

 specific forms, if not identical, at least closely allied to those of the Eastern 

 North American flora. These genera are mostly Miocene ; one. Myrica, is in 

 the Eocene of Black Butte. Leaves described as Popidites from the Creta- 

 ceous of the Dakota group may represent the first forms of Populiis, a 

 genus which becomes more distinctly and more abundantly represented, 

 like Myrica, in the Upper Miocene of Colorado, where the type of Comp- 

 tonia has two or three species. If we add Salix, distinct in the Cretaceous, 

 the Eocene, and the Miocene by species analogous to those of our time 

 and to one of those of the Pliocene, we have passed, without scarcely 

 omitting an}- genus of arborescent plants, the whole series of the generic 

 divisions described in Gray's flora, except the Conifers, which, though 

 absent at some localities. - in the Eocene of Golden, for example, in the 



