1890.] Origin of the Plane-Trees. ` 801 
paper, to introduce here that part of my original paper relating 
to these forms, inasmuch as Ithere dwelt upon them considerably 
more at length. 
* Few as are the living representatives of this genus, it is now 
known that the type played an important rôle in later geologic 
time. More than twenty fossil species have been described, the 
greater part of which are from North American or Arctic strata. 
The American forms mostly occur in what is called the Laramie 
group, which all agree to place very near the boundary line 
between the Cretaceous and the Tertiary formations. The Euro- 
pean, Arctic, and many of our western forms agree well enough 
with living species to leave no room for doubt as to their generic 
affinities, but in the Laramie group there occur some aberrant 
forms which have led to serious difficulties. The most notable 
of these is the Platanus nobilis of Newberry, from the Fort Union 
deposits. Our knowledge of this species is as yet confined to 
what we have been able to derive from the study of a large num- 
ber of very fine leaf impressions. The leaves differ in some 
important respects from those of any living species of Platanus. 
They are usually very large, often measuring over a foot in 
length and width, and instead of having numerous short pointed 
lobes with broad sinuses, they have only three, or at most five, 
lobes, which are large and separated by acute sinuses, the mar- 
gins being entire, or only slightly undulate-toothed. These 
characters give them much the aspect of many species of Aralia, 
and they possess other points of resemblance to that genus. 
They also have the general form of the three-lobed leaves of sas- 
safras. Among the numerous specimens of this type collected 
_ by'me on the Lower Yellowstone, in 1883, there is great variety 
in size, coupled with marked uniformity of shape and nervation. 
The smaller specimens agree in all essential respects with the 
Aralia notata of Lesquereux (Tertiary Flora, p. 237, Pl. xxxıx., 
Figs. 2-4), from Colorado and Wyoming, which he first called 
Platanus dubia (Hayden's Annual Report, 1873, p. 406) [Fig. 1 
of my former paper (Proc. Nat. Mus., Vol. XI., 1888, Pl. xvır.)]. 
“In immediate association with Platanus nobilis, and perhaps 
merely as a state of it, there occurred a form differing chiefly in 
