DESCRIPTIONS OF SPECIES. 45 



essentially that of P. balsamifera, and it can hardly be doubted that we have 

 here the progenitor of one or more of the group of poplars with which I 

 have compared it, and which now grow in the region where these fossil 

 plants were collected. 



The different species of Populus among the Tertiary plants collected 

 by Dr. Hayden are far more generally three-nerved than are the living 

 species which now inhabit this country. In this respect they resemble 

 more the foreign P. alba; and it may be said that the majority of species 

 described in this memoir are more closely allied to the section Coriacege than 

 to the Balsamitse. 



Professor Schimper, in his Paleontologie Vegetale, Volume II, page 690, 

 refers this species to Populus balsamoides Groepp., basing this conclusion 

 upon manuscript information received from Professor Heer. It is impossible, 

 however, to harmonize the discrepancies which exist between the specimens 

 before us and the figures and descriptions of Professor Heer. (Fl. Tert. 

 Helv., Vol. II, p. 18, PI. LIX; LX, figs. 1-3; LXIII, figs. 5, 6; Vol. Ill, 

 p. 173.) In all the figures and descriptions given of P. balsamoides the 

 medial nerve is far stronger than the lateral nerves. These form many 

 pairs, of which the lower reach the margins below the middle of the leaf. 

 On comparing the figure now given it will be seen that the differences are 

 very marked, for the leaf of P. genetrix is practically three-nerved ; at least 

 the midribs and the two chief lateral nerves are nearly of equal strength. 

 The lower pair of lateral nerves may be considered as mere branches of the 

 second pair. From these differences I am compelled to regard P. genetrix 

 and P. balsamoides as distinct species. 



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

 River, Montana, 



Populus litigiosa Heer. 



Pi. Ill, fig. 6. 



Nouv. Mem. Soc. Helv. Sci. Nat., Vol. XXII (1866), p. 13, PI. I, fig. 2; Ills. Cret. and 

 Tert. PI. (1878), PL III, fig. 6. 



The specimen of which the figure is cited above is that of which a 

 tracing was sent by Mr. Meek to Professor Heer, and on which he based 

 his description. This specimen is too imperfect to furnish a full diagnosis 



