fc88 ] PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 41 



The history of this character iu the leaf of the sycamore is thus quite 

 satisfactorily traced as far back as the close of the Mesozoic age, but 

 the type is much older. The next series below the Laramie at which an 

 abundance of vegetable remains is found iu the western portions of the 

 United States is the Dakota group of Kansas and Nebraska, which is 

 usually regarded as Middle Cretaceous, and is about the equivalent of 

 those beds in Europe iu which the most ancient dicotyledonous plauts 

 occur. Throughout this series there are found large-lobed leaves vari- 

 ously referred to Platanus, Ar alia, IAquidambar, Sassafras, Liriodendron, 

 and Aspidiophyllam. The most abundant of these forms has been 

 called Sassafras, or Araliopsis, the latter designation having, however, 

 been generally dropped. It would, of course, be wrong to say that all 

 these forms belong to Platanus; but to predict that they will one day 

 be recognized as interrelated, and as representing the remote ancestry 

 of the plane and the sycamore, can, in the light of our present knowl- 

 edge, scarcely be considered rash. It seems very doubtful whether 

 Liquidambar aud Platanus of the living flora are as dissimilar as would 

 appear from their wide separation in the so-called natural system of 

 classification. That Aralia, Sassafras, aud Liriodendron represent 

 branches of a common trunk from which the former genera have also 

 descended, is much less probable, but not impossible. As regards 

 Sassafras, however, to which genus the greater part of the fossil leaves 

 are supposed to belong, there is no need, I think, of resorting to so 

 violent an assumption, since it is extremely doubtful that the Dakota 

 leaves belong to that type. 



It is a common mistake to look upon the Sassafras as possessing pri- 

 marily a three-lobed leaf. Even those who know that non lobate leaves 

 occur are apt to regard them as abnormal aud the lobed ones as normal. 

 It is a fact well known to botanists that, in the oaks aud many other 

 trees, only the leaves on fruit-bearing branches can be depended upon 

 for the determination of species, and most modern botanists now regard 

 the varying forms of leaf seen ou young shoots and near the base of 

 trees as valuable hints at the probable stages through which the final 

 forms have passed in the history of their development. 



In the Sassafras, after it has attained any considerable size, the 

 greater part of the leaves are elougate and without lobes. These are 

 almost the only leaves found ou flowering or fruiting branches of the 

 larger trees. The lobed leaves occur almost entirely on the lower, 

 barren branches of such trees. Fig. 9, PI. xx, represents a nearly typi- 

 cal leaf from a tree 18 inches in diameter, ou which at least nine-tenths 

 of the leaves were without lobes. Fig. 10, PI. xxi, shows a lobed leaf 

 from the lower portion of the same tree. 



Returning to the nervation, it will be instructive to compare that of 

 the lobed leaf of Sassafras with that of the so-called Sassafras leaves 

 of the Dakota group. Fig. 11, PI. xxi, represents the Sassafras creta- 

 ceum of Lesquereux. 



