50 THE LATER EXTINCT FLORAS OF NORTH AMERICA. 



latticed nervation, and the dentation of the same general character, with 

 the fact that all the specimens are from the same locality, all combine to 

 lead me to consider the two forms as specifically identical. 



Formation and locality: Tertiary (Eocene?). Yellowstone River, Mon- 

 tana. 



Populus polymorphs, Newb. 



PI. XL VI, figs. 3, 4; XL VII, fig. 4; XLIX, figs. 4, 7, 8, 9 [misprmted 1]; LVIII, 



fig. 4. 



Proc. U. S. Nat. Mus., Vol. V (March 21, 1883), p. 506. 



"Leaves petioled, ovate, rounded or slightly wedge-shaped at the base, 

 acute or blunt-pointed at the summit; margins coarsely and irregularly 

 crenate, dentate, or crenate-dentate ; nervation strongly marked, pinnate; 

 in the more elongated forms, about eight branches on each side of the midrib 

 given off at an acute angle; in the broader forms the lower nerves issue at 

 nearly a right angle; the upper ones at an angle larger than in the preceding 

 form." 



The leaves of this tree are the most numerous of all represented in the 

 collection from Oregon made by Rev. Thomas Condou, several hundred in 

 greater or less completeness being included in the specimens which have 

 been passed in review. They show a marked diversity of form, some being 

 long ovoid or elliptical, rather pointed at base and summit; others ovoid or 

 roundish with a rounded base ; some are light and delicate, others have strong 

 nerves, and evidently were thick and leathery in texture. More generally 

 a base similar to that on PI. XLIX, fig. 9, accompanies a summit coarsely 

 dentate or crenate. 



It is with some hesitation that this leaf has been referred to Populus, 

 but it presents greater affinities in nervation and marginal markings with 

 this group than any with which they have been compared. The general 

 aspect of the leaf represented on PI. XLVI, fig. 4, is quite that of some of 

 the poplars, particularly of the group represented by the abele (P. alba, L.), 

 while the specimens figured on PL XLIX, figs. 4 and 7, and PL XLVII, 

 fig. 4, are so different from the prevailing style of poplar leaves that the 

 propriety of referring them to this genus seems questionable. There are, 

 however, connecting links between all these different forms, and the general 



