GENERAL CONCLUSIONS. 43 



describes thirty-two species, and quotes, in the comparative examination, 

 most of those known in Europe from the same formation. Not one of them, 

 however, offers a close affinity to the plants of the Chalk Bluffs. This 

 difference is explainable by the likeness of the characters of the Plio- 

 cene species to those of the present time, — a relation which reduces the 

 affinities to local or geographical limits, as they are now. The circum- 

 scriptions are wider, or the geographical areas less distinctly fixed in 

 older o-eolo^ical divisions, and thus the flora of the Chalk Bluffs has 

 some Miocene species identifiable in Europe, but none of its Pliocene 

 as yet. 



On another side, in coining nearer to the present period the vegetable 

 forms become more and more similar to those of our time, some being 

 apparently identical. But it is very difficult to make out positive iden- 

 tity from the characters of leaves only. The identity is probable, evi- 

 dent to the eyes of the observer; but it cannot be proved. For species 

 of this kind a derivative appellation, indicating supposed identity, like 

 pscado or the terminative ites, seems more appropriate. The authors of 

 the "Flora of Maximieux" append to the specific name the epithet pliocene, 

 and thus have Pojmlus alba (pliocemca), etc. 



The Miocene relation of the flora of the Chalk Bluffs is indicated by 

 a few identical species : Fagus Antipofi, Heer, described from the Miocene 

 of Alaska, of France, and of Arctic Russia ; Populus Zaddachi, Heer, pre- 

 dominant in the Upper Miocene of the Baltic, and found also in the same 

 formation of Alaska, Greenland, and Spitzbergen ; Ficus tilicefotia, Al. Br., 

 present in the whole Miocene of Europe as far north as (Eningen, and 

 in the North American from the Lower Lignitic measures, which I con- 

 sider as Lower Eocene, through the different stages of the Tertiary ; Ardlia 

 Zaddachi? Heer, whose identification is as certain as it can be made in 

 the comparison of our specimens with the mere fragment which repre- 

 sents this species from the Baltic Miocene. Besides this, we find a marked 

 affinity between Qitercus elcenoides and Q. elcena, Ung., a common Miocene 

 species of Europe ; SaUz elliptica, related to S. varians, Goepp. ; Ficus sor- 

 did,/, closely allied to, if not. identical with, F. Groenhndica, Heer, of Green- 

 land; F. mwrophyUa, which seems a mere diminutive form of F. plamcostcda, 

 a common species of the Lower Lignitic of the Rocky Mountains; Aralia 

 Whitneyi, related to A. affinh of the group of Evanston, Middle or Upper 

 Eocene ; Acer wiptidadaltoit. related to Acer vUifolium of (Eningen in a 



