MUSEUM OF COMPARATIVE ZOOLOGY. 47 
with the nervation of the species as figured in U. 8. Geol. Surv. of the Terr., 
VIL., Plate XXIV. fig. 5. The size is also the same. In this fragment the 
basilar border is abruptly turned downward, and slightly decurrent to the 
petiole, as in P. subrotundata, fig. 8 of the same plate. The petiole is inflated 
below the border, as in some species of Ficus. But that is apparently a result 
of compression. 3 specimens. 
32. Populus monodon, Lx. 1 specimen. 
33. Populus mutabilis, Hr. 1 specimen. 
34. Populus Gaudini, Heer. A small leaf, ovate in outline, narrowly long- 
acuminate, truncate at base ; pinnately nerved ; nerves thin. 
The leaf is of the same size as that of the species in Heer, Fl, Tert. Helv., 
Plate LXIV. fig. 3. It has also the same form ; except that it is narrowly 
acuminate, like fig. 6 of the same plate and same species. The leaf is 5 cm. 
long, 34 cm. broad near its base, and abruptly curved to the slender petiole. 
1 specimen. 
35. Populus Zaddachi, H. 1 specimen. 
36. Populus arctica, Heer. The species is represented by a number of speci- 
mens, all small leaves, 2-4 cm. long, crenulate on the borders, more or less 
enlarged transversely, five-nerved from the base, coriaceous. 
Except that the leaves are generally more distinctly crenulate, nothing in 
the characters indicates a difference from those which I have figured in U. S. 
Geol. Surv. of the Terr., VII., Plate XXIII. and Plate XLVI., or of those in 
Heer, FI. Arct., L., Plate IV. fig. 6a. The nervilles are generally strongly 
marked, 15 specimens. 
37. Populus Nebrascensis, Newby. The species is represented by a very large 
number of specimens, some of them with the characters indicated by the author 
in “Illustrations and Notes on the Extinct Flora of North America,” p. 62, Plate 
XII. figs. 4, 5, while others are definite varieties, which could be considered as 
species closely allied to P. arctica, P. Zaddachi, and P. Richardsoni of Heer. 
I have separated the varieties by short diagnoses. The true P. Nebrascensis of 
Newberry has the teeth of the borders unequal, always obtuse. It differs from 
P. arctica by the absence of transverse nervilles, and the leaves longer, generally 
ovate-lanceolate, nerved to the base. 145 specimens. 
38. Populus Nebrascensis, var. grandidentata, Lx. Leaves broader, rounded 
and undulate toward the base ; borders cut from the middle upward in large 
deltoid obtuse gradually longer teeth. Some of the leaves are subtruncate at 
apex with long irregular teeth ; others are rapidly narrowed to an obtuse apex. 
The nervation is the same in the varieties as in the normal form, 3—5 palmate 
from the base, with the inner pair of primary nerves curved inward and ascend- 
ing to near the apexes and the secondary ones at a great distance from the base. 
85 specimens. 
39. P. Nebrascensis, var. rotundata, Lx. Much like the preceding, differing 
from it by the broader leaves, broadly round and enlarged at the base. The 
teeth are obtuse and large. 48 specimens. 
40. P. Nebrascensis, var. acute-dentata, Lx. Leaves oval, narrowed at base, 
