MUSEUM OF COMPARATIVE ZOOLOGY. 47 



with the nervation of the species as figured in U. S. Geol. Surv. of the Terr., 

 VII., PUite 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. subrohmdata, 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 comjDression. 3 specimens. 



32. Pupulus 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, 3-^ 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, Fl. Arct., I., Plate IV. fig 6 a. 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, vav. 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, 



