458 SYSTE]!i£ATIC PALEONTOLOGY 



primceva of Heer ^ from the Kome beds (Barremian) of Greenland, still 

 remaining one of the oldest Imown dicotyledons of undisputed identity. 



Two other species described by Heer from the Arctic Tertiary, Populus 

 Zaddachi and Populus arctica have the further distinction of having 

 been found fossil in latitude 81° 46' on the north shore of Grrinnell 

 Land. 



The Potomac species comes next after Populus primceva in point of 

 age. With the dawn of the Upper Cretaceous a number of species appear 

 including four in the Atane beds of Greenland, ten in the Dakota sand- 

 stone and four or five in the Raritan, Tuscaloosa and Magothy forma- 

 tions. The first European species appear to be of Senonian age. After 

 the close of the Cretaceous the genus expanded suddenly, especially in. 

 America, over a score of species being tnown from the Fort Union beds 

 and many from the Arctic Tertiary. It remained cosmopolitan in the 

 northern hemisphere throughout the Tertiary and Recent periods, and 

 several of the existing species, which number in all about 25 forms, are 

 f)resent in the Pleistocene deposits of both America and Europe. 



Populus potomacensis Ward 

 Plate LXXXI, Figs. 1-le 



Populus potomacensis Ward, 1895, 15th Ann. Rept. U. S. Geol. Survey, p. 



356, pi. iv, figs. 1-3. 

 Populus auriculata Ward, 1895, 15tli Ann. Rept. U. S. Geol. Surv., p. 356, 



pi, iv, fig.. 4. 

 PopulopTiyllum menispermoides Ward, 1906, in Fontaine, Mon. U. S. Geol. 



Survey, vol. xlviii, 1905, p. 498 (pars), pi. ex, fig. 2 (non figs. 3, 4). 

 Populus auriculata Ward, 1906, in Fontaine, Mon. U. S. Geol. Surv., vol. 



xlviii, 1905, p. 499, pi. ex, fig. 5. 

 Populus potomacensis Ward, 1906, in Fontaine, Mon. U. S. Geol. Surv., vol. 



xlviii, 1905, p. 500. 



Description. — ^Leaves of small size, orbicular to ovate in general out- 

 line, with an obtusely pointed apex and a broad, deeply cordate base. 

 2 cm. to 5.5 em. in length by 2;2 cm. to 4 cm. in greatest width, which 



^ Heer, Fl. Foss. Aret, Band iii, Abth. ii, 1874, p. 88, pi. xxiv, fig. 6. 



