54 



kota group are leaves of Dicotyledonous, representing the three, di 

 visions of this class, and, what is more remarkable, the genera to which 

 belong most of the living arborescent plants of this country and of our 

 present climate. If what maybe called positive characters of the genera — 

 the flowers and the fruits — are not ascertainable from fossil fragments, it 

 is at least impossible to deny the intimate relation of most of the leaves 

 of the Dakota grouj) to the genera to which they have been referred in 

 their descriptions. 



Beginning by the Apetalous, we have first Idquidanihar leaves so simi- 

 lar to those of our sweet-gum tree, L. styracijluuin, by form and 

 nervation, that in comparing the fossil leaves with those of our living 

 species, no difference whatever can be remarked but in the entire borders 

 of the fossil ones. They are more or less serrate- crenulate in the living 

 species, as also in L. Europeum of the Miocene of Europe. But some 

 species of the same formation, and considered by authors as referable 

 to this genus, have leaves with entire borders. Even Gaudin, in his 

 memoirs on the fossil leaves of Tuscany, figures as L. Uuropeum, three 

 leaves, one of which, with entire borders, PI. V, Fig. 3, looks like a 

 counterpart of our Fig. 2, of PL III, the lateral nerves being marked as 

 branches of the second pair of nerves, just as it is in our Cretaceous leaves, 

 and not emerging from the top of the petiole as in the leaves figured by 

 Heer under the same name. Gaudin accounts for the entire borders of this 

 leaf by the supposition that the denticulation cannot be remarked on 

 account of the coarseness of the stone where the leaves are imbedded. 

 We could give the same reason or admit such a supposition, but the 

 forms of the leaves of this genus are so distinct that the difference in the 

 more or less serrate borders cannot prevent their generic identification. 

 The leaves of the Cretaceous species are, especially by their truncate base 

 and their general outline, rather related to those of our L. styracijluum, 

 than to those of the Asiatic form, L. orientale. These are the two only 

 living species of Liquidamhar, with ijalmately-lobed leaves. 



The history of this Genus, its origin, and the present distribution of its 

 species, offer with that of Platanus a coincidence worth remarking. Both 

 appear first in the Dakota group ; both pass through the Tertiary forma- 

 tions of Europe in different modifications, and both, too, have each for 

 essential representatives of the present flora an oriental and an occiden- 

 tal form. In Asia, Liquidamljar orientale Siud Platanus orientale; in our 

 country, L. styraciflimm and P. occidentalism of which the Mexican forms 

 are mere derivations. No species of Liquidamhar has been as yet rec- 

 ognized in our Kortli American Tertiary formations. Two species widely 

 distributed are described with numerous varieties from the Tertiary of 

 Europe. 



The numerous leaves which are referred to the genus Po2>MZt#es are com- 

 parable to those of the different species of poplars now inhabiting the 

 north part of this continent. The relation is not positive, however. 

 They essentially differ by the peculiar disposition of the secondary veins 

 to run straight to the borders, a kind of nervation (craspedodrome) re- 

 marked in the leaves of the beech, and in very few indeed of the leaves, 

 with entire borders of our present arborescent vegetation. Leaving out 

 this difference and considering the apparent affinities, we have Populus 

 Haydenii, referable to the section of the Marginate to which belong our 

 P. monilif era, Qjnd some of its numerous varieties, P, canadensis, P. aiigu- 

 lata, t&c, and related also by its nervation and its peculiar denticulation 

 to P. candicans of ours, and P. halsamoides of the Miocene. Populites cyclo- 

 pJiylla and P. lancastriensis have typical affinity by their nervation and 

 the enlarged form of the leaves with Populus latior, while Populites elegans 



