64 THE LATER EXTINCT FLORAS OF NORTH AMERICA. 



and the coarse, double-crowded dentation of the upper portion of the leaf. 

 They are also separated by these characters from the numerous other spe- 

 cies of the genus mentioned by Professor Heer as found in the Tertiary of 

 the northern part of this continent, B. macropliytta (Fl. Foss. Arct, Vol. I, 

 p. 146, PL XXV, figs. 11-19), B. prisca Ett. (Heer, Fl. Foss. Arct., Vol. 

 I, p. 148, PL XXV, figs. 20-25; Fl. Foss. Arct, Vol. II, Abth. II, p. 28, 

 PL V, figs. 3-7). They bear a closer resemblance to the leaves of B. 

 ostrycBfolia Sap. (Fl. Foss. Sezanne, p. 345 [57], PL XXV [IV], fig. 8), and 

 B. Sezannensis Wat. (PL Foss. Bass. Paris, p. 130, PL XXXIV, fig. 6); 

 but both these species are crenato-dentate, while in the leaves before us 

 the teeth are acute. Among living species this may be compared with 

 B. lenta Willd., but is narrower and the marginal dentation is less uniform. 



The leaf figured on PL XLVII, fig. 5, differs somewhat from those 

 which have been here associated with it, in its more rounded base, coarser 

 dentation below, more open and opposite nervation, and it may represent 

 another species. Part of these differences, however, is probably due to 

 difference in the preservation of the marginal dentation. 



Formation and locality: Tertiary (Miocene). Bridge Creek, Oregon. 



Betula heterodonta Newb. 

 PI. XLIV, figs. 1-4; XLV, figs. 1, 6. 

 Proc. U. S. Nat. Bins., Vol. V (March 21, 1883), p. 508. 



"Leaf 2 to 4 inches in length, long petioled, ovate, acuminate, rounded 

 at the base; margins coarsely and irregularly serrate, the principal denticles 

 receiving the terminations of the nerve branches; the sinuses between these 

 sometimes plain, sometimes set with a few small teeth; nervation delicate, 

 about eight branches given off from each side of the midrib." 



The collection from Oregon, made by Rev. Thomas Condon, contains 

 a large number of leaves belonging to this species. These present consid- 

 erable variety in size, as will be seen in the figures. There is also some 

 diversity in the degree of denticulation of the margin. The examples 

 which show this best among those figured are PL XLIV, fig. 2 ; PL XLV, 

 fig. 1. Here we see the lateral nerve branches running into prominent 

 teeth of the margin as in many other species of Betula, such as B. nigra L., 

 B. Blancheti Heer, but the sinuses between these large teeth are sometimes 



