Maryland Geological Suevet 459 



is in the basal half of the leaf. Margin crenulate, entire in the basal 

 sinus. Petiole stout. Midrib of medium calibre. Primaries 3 to 7 in 

 number decreasing in calibre outward, inserted at the apex of the petiole, 

 curving upward, camptodrome. 



This characteristic and handsome little species is abundant in the 

 Mt. Vernon clays to which locality it is thus far confined. It shows 

 considerable variation in the amount of elongation, some specimens 

 being relatively narrow with a consequently more pointed apex and more 

 acute basal sinus, but there can be scarcely any doubt that the forms 

 figured were all borne by the same tree, although they constituted a 

 part of three different species of Professor Ward, as enumerated above. 

 The outlines of Populus leaves in general vary considerably in the same 

 species and on the ^same tree. The venation on the other hand is more 

 conservative, and it is on the basis of these well-known facts that the 

 present treatment is based. 



Occurrence. — Patapsco Formation, Mt. Vernon and White House 

 Bluff, Virginia. 



Collection. — U. S. National Museum. 



Genus POPULOPHYLLUM Fontaine 

 [Mon. U. S. Geol. Survey, vol. xv, 1890, p. 311] 



This genus named to indicate a supposed but altogether doubtful 

 relationship with the genus Populus is characterized as follows by its 

 describer : 



"Leaves rotundate; petiole thick and apparently somewhat succulent; 

 nerves tending to a radiate grouping from the summit of the petiole; 

 midnerve not much surpassing in strength the nerves which go off on 

 each side of it." 



As here understood the genus embraces two species, one with an entire 

 or slightly undulating margin and the other with a coarsely dentate 

 margin. It may be doubted whether the two are congeneric, the former 

 certainly shows but the slightest aflBnity with Populus and suggests the 

 genus Ohondrophyllum. Similar remains from the Lower Cretaceous 



