DESCRIPTIONS OF SPECIES. 47 



Populus Nebrascencis Newb. 



PI. XXVII, figs. 4, 5. 



Anu. X. Y. Lye. Nat. Hist., Vol. IX '(April, 1868), p. 62; Ills. Cret, and Tert. 

 PI. (1878), PI. XII, figs. 4, 5. 



"Leaves long-petioled, 2 to 3 inches long, ovate, pointed, regulai-ly 

 rounded at the base, coarsely and irregularly toothed except near the base 

 where the margins are entire; nervation strong, radiating from the base of 

 the leaf; medial nerve straight, simple (or supporting very small nerves), 

 except near the summit, where two or three larger branches rise from it; 

 lateral nerves, two pairs on each side, springing from a common point of 

 origin; lower pair arched upward, nearly parallel with the margin of the 

 leaf, to which they send off one or more simple branches; second pair of 

 laterals diverging from these at an angle of 30 degree's, arching upward, 

 and running parallel with the midrib, terminating in the margin near the 

 summit, each giving off about three exterior branches, which curve upward 

 and terminate in the dentations of the border." 



This species, hj its general form and nervation, approaches closely to 

 P. smilacifolia, but the base is rounded (sometimes slightly wedge-shaped), 

 never distinctly cordate; the superior lateral nerves are not quite so much 

 drawn together toward the summit, and the margins are differently and 

 much more coarsely dentate. 



A large number of specimens of this species present constant and 

 distinctive characters. They exhibit considerable variation in size, being 

 from 1 to 3 inches in length, but in form, nervation, and marginal dentation 

 they are alike. 



These specimens, from the collections made by Dr. F. V. Hayden, are 

 derived from different localities, and without doubt represent a distinct 

 species which was spread over the Tertiary continent. 



By the character of the impressions left on the stone, as well as by the 

 coarse and unequal dentation of the margins, we may infer an affinity 

 between this and the downy-leafed poplars of the present epoch, such as 

 P. alba of Em-ope, etc., while in the smooth surface and finely denticulate 

 or entire margin of P. smilacifolia we have evidence of resemblance to P. 

 tremuloides. 



