INTEODUCTION. 7 



authors as uncertain in regard to their generic relation merely on account 

 of their entire borders. The form of the leaves, however, especially as 

 figured (pi. xiv, fig. 3), with the lobes slightly enlarged above the sinuses, 

 then gradually narrowed to a slightly obtuse point, and the nervation also, 

 have the same character as those of the living Liquidamhar Styraciflua. 

 It is true that the four species of this genus known in the present flora 

 have serrate borders of leaves. But three fossil species represented by 

 leaves with entire borders have been described as Liquidamhar from the 

 Tertiary of Europe; and, though this reference is more or less hypothetical 

 and controverted, it shows, nevertheless, that botanists of high standing — 

 Unger, Watelet, Massalongo — have considered it, at least, as probable. It 

 is easily seen that the leaves of Aralia Towneri (pi. vi,fig.l4) have a relation 

 in shape or general outline to those of Liquidamhar integrifolium, and this 

 apparent similarity can but suggest the possible relation of all these and 

 like forms to the genus Aralia. I may admit this relation as probable for 

 the two leaves figured in "Gret. Fl.," pi. xxix, figs. 8 and 9, which are com- 

 parable, by their primary nervation, to those of Aralia concreta (pi. ix, 

 figs. 3, 5). But though we have now a large number of specimens refer- 

 able to diverse Araliaceous types, there is none as yet with leaves divided 

 into lanceolate acute lobes like those which are figured in pi. ii, "Cret. Fl.,'' 

 and with five primary nerves from the base. The reference of these leaves 

 to Sterculia has been proposed also, from analogy of forms. But according 

 to the definition of this genus as I admit it for the fossil leaves of the 

 Dakota Group, I refer to it merely tripartite leaves with narrow linear 

 lobes, comparable to those of Sterculia lahrusca, like those of the few species 

 described in this volume. 



A number of vegetable remains of the Cretaceous are evidently refer- 

 able by their characters to Popidus. The only fragments of dicotyledonous 

 leaves recognized by Heer, in the specimens which he studied from the 

 Lower Gretaceous formations of Greenland (Kome), represent a Popidus. 

 appropriately specified by the name of P. primceva. From a higher stage of 

 the same Gretaceous formation of that country (Atane) the celebrated Swiss 

 paleontologist has described four other species of Populus. In his " Phyllites 

 Gretacees du Nebraska," and from specimens of the Dakota Group, he has 

 recognized Populus litigiosa, Populus (?) Deheyana, and another species still, 



