GENERAL CONCLUSIONS. 49 
very difficult to fix. Péatanus 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. Gulielne of the Miocene of Carbon; and by 
the species of the Pliocene of California. It is the same with Jugdans and 
Carya, not positively recognized, however, in the Cretaceous, but already 
present by different species in the Kocene 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 Carya, 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. angustiloba recalls our Q. falcata. Highteen forms of Quercus, recognized 
in the Lignitic Tertiary flora, show to those of our time an analogy becom- 
ing still more distinct by the species of the Pliocene. Custanea 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 J. sy/- 
vatica and F. ferruginea. Corylus is Kocene. Dr. Newberry has described 
from the Fort Union group leaves of this genus under the specific name 
of @. 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, Alius, Betula, Carpinus, in 
specific forms, if not identical, at least closely allied to those of the Kastern 
North American flora. These genera are mostly Miocene; one, Myrica, is in 
the Eocene of Black Butte. Leaves described as Populites from the Creta- 
ceous of the Dakota group may represent the first forms ef Populus, a 
genus which becomes more distinctly and more abundantly represented, 
like Myrica, in the Upper Miocene of Colorado, where the type of Comp- 
toma 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 any 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 
