38 THE LATER EXTINCT FLORAS OF NORTH AMERICA. 



above the middle, or in the lobes, when lobes are developed; from these 

 spring three or four branches on the outside, which, simple or branching, 

 terminate in the scallops of the border. The tertiary nervation, shown 

 very distinctly in some of the specimens, forms a network similar to that of 

 the leaves of living species of Populus, of which the areolae exhibit con- 

 siderable diversity of form and size, being polygonal, with a roundish 

 outline, or quadrangular." 



Collected by Dr. F. V. Hayden. 



The general aspect of these leaves is much like that of some of the 

 living maples, but they are less distinctly trilobate. The crenation of 

 the margin is coarse, irregular, and obtuse or rounded, as is usually the 

 case with the leaves of a group of poplars, the leaves of which in other 

 respects most resemble these. The surface is, in many specimens, some- 

 what roughened, as though in the living leaf it was canescent; also a com- 

 mon character among poplars, but rare or unknown among maples. The 

 leaves of the maples are generally thin, and the network of the tertiary 

 nerves is remarkably fine and uniform, affording a reliable generic charac- 

 ter. This is visible in the leaves of all the recent maples, and is beau- 

 tifully shown in the impressions of the leaves of A. pseudoplatanus, 

 given in Ettingshausen and Pokorny's Physiotypia Plant. Austria, PI. XVII, 

 fig. 10. 



Among fossil species this perhaps resembles most P. leucophylla (Foss. 

 Flor. v. Gleichenberg, Denkschrift, k. k. Acad. Wien., Vol. VII (1854), 

 p. 177, PI. IV, figs. 6-9), but is much more distinctly crenate-toothed on 

 the margin. The teeth of P. leucophylla are either obsolete or remote and 

 acute, making a sinuate-dentate margin. 



Formation- and locality: Tertiary (Fort Union group). Fort Union, 

 Dakota. 



Populus cordata Newb. 



PI. XXIX, fig. 6. 



Ann. N. Y. Lye. Nat. Hist., Vol. IX (April, 1868), p. 60; Ills. Cret. and Tert. PL 

 (1878), PI. XIV, fig. 6. 



"Leaves orbicular or round heart-shaped, deeply cordate at the base; 

 margins strongly toothed, except the inner border of the lobes of the base; 



