INTEODUCTION. 



In regard to the distribution of Populus, to which are referred the 

 most ancient dicotyledonous leaves known as yet, from the Lower Cre- 

 taceous of Greenland, the genus has, as said above, three species known 

 already in the Upper Cretaceous of that same country, and five or six in 

 the Dakota Group. It has, however, not been remarked in any Cretaceous 

 Flora of Europe. It is not mentioned in the review of the genera repre- 

 sented by the, as yet, undescribed species of Aix-la-Chapelle,^ and no form 

 even distantly related is described in the Lower Paleocene Flora of Gelin- 

 den. It has, however, one species in the Eocene Flora of Sezanne, and 

 increases in the number of its representatives in all the stages of the 

 European Miocene. As far as we know it, till now, it has few species in 

 our Lower or first American Tertiary Group — the Eocene; has a large pro- 

 portion, eight per cent, of the species, in the Evanston Group; still more, or 

 twelve per cent., in the Miocene of Carbon, and is present in the second, 

 the Green River Group in four species, three of them of peculiar types, 

 one of which is very abundant. 



The presence of Willows (Salix) in the Flora of the Dakota Group 

 cannot be controverted, though neither seeds nor scales of this genus 

 have been found as yet. As it is seen in "Cret. Fl.," p. 60, pi. v, figs. 

 1-4, I have described as referable to one species only a number of leaves 

 somewhat different in size and shape. As the specimens representing 

 them are from the same locality, and as I recognized upon some numerous 

 fragments of leaves a unity of character, size, form, and even texture and 

 color, I considered them as mere varieties of leaves of the same tree. Dr. 

 Newberry has, from the same formation, four species which, he says, he 

 has chosen to regard as distinct, for geological convenience. No Salix has 

 been recognized as yet in any stage of the Cretaceous of Greenland; but 

 one species, Salicites Hartigii, Dkr., is from the Quader-sandstein of Ger- 

 many, and another, Salix Goetziana, Heer, from Quedlinburg. The genus 

 is therefore sparingly represented in Europe and North America in Cre- 

 taceous Floras which are considered as nearly synchronous. 



The other genera of the Amentacew, Setula, Alnus or Alnites, Myrica, 



•Dr. M. Debey has recently publiBhed a fine memoir on Bome querciform leaves found in the sand rooks 

 of Aix-la-Chapelle, Rhenish Prussia. 



