48 THE LATER EXTINCT FLORAS OF NORTH AMERICA. 



There is no fossil species for which this can well be mistaken. Some 

 of the forms of P. crenata Unger (Foss. Fl. Sotzka, p. 166 [36], PI. 

 XXXVI [XV], figs. 2-5) resemble these leaves, but they are not so dis- 

 tinctly radiate nerved. Unger represents the teeth of the margin as more 

 acute, and more like those of P. tremula, with which he compares his fossil 

 species. 



Some varieties of Populus Zaddachi Heer (Fl. Tert. Helv., Vol. Ill, 

 p. 307; Fl. Foss. Arct, Vol. I, p. 98, PI. VI, figs. 1-4; XV, fig. lb) are 

 somewhat like this species, and it has been suggested by Mr Lesquereux 

 that they are identical; but in all the figures of that species published 

 the margins are serrate-dentate, whereas in the leaves before us they are 

 much more closely crenate-dentate; also most of the leaves are cordate at 

 the base, and this is a feature given by Heer in his description, but among 

 quite a large number of the leaves of P. Nebrascencis which have served as 

 a basis for the specific description, the form is ovate, the base rounded, some- 

 times a little produced, but never cordate or even emarginate. 



Formation and locality : Tertiary (Eocene?). Banks of the Yellowstone 

 River, Montana. 



Populus nervosa Newb. 



PL XXVII, figs. 2, 3. 



Ann. N. Y. Lye. Nat. Hist., Vol. IX (April, 1868), p. 61; Ills. Cret. and Tert. PL 

 (1878), PL XII, figs. 2, 3. 



"Leaves rounded in outline, margins nearly entire, or slightly serrate 

 at the base, sharply but not deeply toothed on the sides, on the summit 

 strongly doubly serrate, with a tendency to become three-lobed; nervation 

 strongly marked and crowded; basal nerves springing from the midrib 

 above the margin, given off at an angle of 30 degrees or more, reaching the 

 margin above the middle, where they terminate in the most prominent teeth 

 or lobes; from these basilar nerves are given off five or six strong lateral 

 nerves, which arch upward and, more or less forked, terminate in the mar- 

 ginal teeth ; above the basilar nerves three or four pairs of strong lateral 

 nerves are given off from the midrib, which run parallel with the basilar 

 pair, and terminate, like them, in the compound teeth of the upper margin. 

 The lateral nerves are connected by numerous strong secondary nerves, 



