42 TALEONTOLOGIC HISTORY OF PLATAXUS. 



The disposition of the nerves proceeding from the midrib to the sinuses 

 is remarkably uniform in all Sassafras leaves, as any one cau prove by 

 observation ; yet it is here that the widest difference is seen in the fos- 

 sil forms. These, however, bear some resemblance to the fossil forms 

 of Platanus and to those called Aralia, which are probably of the same 

 type. 



None of the supposed Sassafras or Liquidambar leaves of the Dakota 

 group show the basilar expansions that occur iu some of the species of 

 Platanus of later age, but in the anomalous form which has been called 

 Aspidiophyllum something analogous to them is seen. Figs. 12 and ]3J 

 PI. xxii, represent the Aspidiopliyllum trihbatum Lx., the first of which 

 shows the three lobes and nervation, while in the second the expansion 

 at the base is somewhat lobed. 



It is remarkable that certain of the remote ancestors of onr familiar 

 tulip tree are found to approach this same type, at leastiu general form, 

 and one species formerly referred by Professor Lesquereux to that genus 

 (Liriodendron), but finally classed as an Aspidiophyllum, has the en- 

 larged base of blade with narrow neck in singular imitation of the Plat- 

 anus leaves of the Lower Yellowstone Valley. 



As regards Aralia, none of the Cretaceous forms thus far found pos 

 sess this feature, but one of the species which I have myself called by 

 that name, the beautiful Aralia digitata from the Fort Union deposits 

 (fig. 15, PI. xxii), shows a decided tendency in this direction, and 

 though small and deeply lobed into five narrow digits curiously like 

 fingers of the human hand, I can see nothing in the general nervation, 

 dentation, or form that differs essentially from those of the largest 

 leaves of Platanus nobilis from rocks of the same age. 



The American origin of our sycamore was long denied by Willdenovius 

 ami other European botanists, and was only rendered certain by its di 

 covery in a fossil state by Professor Lesquereux in a late deposit of the 

 Mississippi Valley. Specimens were sent to that great authority on 

 these subjects, Dr. Oswald Deer, of Zurich, who could find no characters 

 by which to distinguish the fossil from the living form and who regarded 

 this as a final settlement of the question.* But we have now learnei 

 that not only this most abundant species, not only the greater number 

 of the living species are American, but that the genus itself, the entire 

 type of vegetation to which the planes belong, is American, and tl a I 

 numerous and strange archaic forms of this type not only formed the uin 

 brageous forests on the shores of the great inland Laramie sea where the 

 Rocky Mountains now stand, but also those of theocean at a time when 

 it still pushed its arms northward across what are now the great plains 

 of Texas, Colorado, and Wyoming. 



'Bulletin de la soci£t6 Vandolse, Tome V, Lausanne, ltfoe 1 , p. 144. 







