61 



the list of the genera of the Dakota group. By their present distribu- 

 tion, our P. serotina and F. Virginiana indicate an extreme power of life, 

 or of resistance to climatic changes, both being the only arborescent 

 species of this continent, with a range of 30"^ to 35° in latitude, and both, 

 too, being found everywhere, on every kind of ground ; the one as a shrub 

 along the banks of streams, the other as a fine tree in our woods. And, 

 also, we have in our P. CaroUnimui, a shore tree of the South, a species 

 whose coriaceous entire leaves recall the essential characters of those of 

 the Dakota group. Three species of this genus are described from the 

 Tertiary of Europe, none as yet from ours ; but it is probable that fossil 

 remains referable to it will be found hereafter, as it has in our present 

 flora a larger number of species than in that of Europe, or of any other 

 part of the world. Of the species described by De Oandolle, fourteen 

 are North American, five European ; four species belong to Japan, &c. 



Resuming in a few sentences the above remarks, we find : That the 

 Dicotyledonous flora of the Dakota group represent species referable to 

 the genera Liquidamhar, Populus^ Salix, Betula, Myrica, Querciis, (in two 

 of its principal types,) Ficus, Flatanus, Laurus, Sassafras, Cinnamomumy 

 Diospiros, Aralia, Magnolia, lAriodendron, Menispermum, Negundo or 

 Acer, (!) Palinrus, Rhus or Juglans, ("?) and Prwwws _; or, merely considering 

 the affinities to our present flora, of twenty genera, seventeen of which 

 are those to which belong the species of our trees and shrubs which have 

 the more general and the widest range of distribution. Indeed, all our 

 essential arborescent types are there, except those which are marked by 

 serrate or doubly-serrate leaves : TiUa, ^sculus, all the serrate Bosacece; 

 HamameUs, Fraxinus ; the Urticinece ; Flanera, TJliaus, Celtis, Morns; 

 and of the Amentacece, the serrate Betula, Alnus, Ostrya, Carplmis, Cory- 

 lus, Carya, &c. 



This enumeration exposes the general fades of the leaves or of the 

 flora of the Dakota group, viz, integrity of the borders and coriaceous 

 consistence of the leaves. The borders, if not perfectly entire, are merely 

 undulate or obtusely lobed. There is only one exception to this in that 

 peculiar short denticulation with outside turned teeth, which is marked, 

 exactly of the same kind, in PoindUes Saydenii, P. flabellafa, Flatanus 

 JSfeivherryi, Quercus Mudgii, and the fragments described as Fhyllites betu- 

 loefolius. This mode of division of the borders of leaves is very rare in 

 species of our present times, except, perhaps, in some leaves of poplars. 

 A single of the leaves of the Dakota group Quercus primordialis has the 

 borders distantly serrate or marked by teeth turned upward. There is 

 also in the flora of the Eocene of the Eocky Mountains a marked x)repon- 

 derance of leaves with entire borders. The serrate leaves appear in the 

 Miocene, with Acer, Alnus, Corylus, and become predomiuent in the 

 Pliocene of California, where Ulmus, Flanera, Celtis, Carya abouncl, 

 though these genera are not more in the flora of the Pacific slope. 



But of the detailed correlation of the flora ©f the Dakota group with 

 that of the subsequent geological epochs of this continent, I will say 

 nothing more now until the materials on hand are definitely described 

 and figured for comparison. 



There is little to say on the analogy of this Cretaceous flora of ours 

 with that of any of the Cretaceous groups of other countries. Of the 

 Ferns, we have a GleicJievia Kurriana (f) apparently identical with the 

 form described under this name from Moletin. Of the Conifers, there 

 is a specimen doubtfully referable to Geinitzia and of the Dicotyledonous, 

 an Aralia leaf, also closely allied to species of the same locality. With 

 the flora of the Quader Sanstein of the Hartz, that of the Dakota group 



