46 THE LATER EXTINCT FLORAS OF NORTH AMERICA. 



of the species or to afford comparison with the other fossil plants with which 

 it is associated. 



It is evident, however, that the general form of the leaf and the char- 

 acter of the nervation are similar to those of P. cyclophylla Heer, but it would 

 seem that the margin is somewhat waved, and the nervation is rather more 

 open than in the larger specimens of the species with which I have com- 

 pared it. The basal pair of nerves also form a slightly greater angle with 

 the midrib, and branches given off from them below are longer, supplying a 

 broader expanse of the leaf. Like several of the other less common leaves 

 of the Dakota group, these must remain as somewhat doubtful material imtil 

 further collections shall add to our knowledge of them. 



Formation and locality: Cretaceous (Dakota group). Blackbird Hill, 

 Nebraska. 



PoPULUS MICROPHYLLA Newb. 



Pi. III. fig. 5. 



Ann. N. Y. Lye. Nat. Hist., Vol. IX (April, 1868), p. 17; Ills. Cret. and Tert. 

 PI. (1878), PI. Ill, fig. 5. 



" Leaves very small, scarcely an inch in length, roundish in outline, 

 somewhat wedge-shaped at base, where they are entire; the upper part of 

 the leaf rounded and deeply toothed, teeth conical, acute or slightly 

 rounded at the summits; nerves radiating from the base, branching above, 

 the branches terminating in the dentations of the margin." 



This very neat species, from the collection made by Dr Hayden, 

 might be supposed to be only a form of P. elliptica, with which it is asso- 

 ciated, but a number of specimens of each show no shading into each 

 other, and it is scarcely possible that so wide a variation of marginal denta- 

 tion should exist in the same species. Although the leaves of P. elliptica 

 are two or three times as large as those of the species under consideration, 

 the teeth of the margins are less than half the size and are of a different 

 type, being inclined upward, the sides of each tooth of unequal length, 

 while the dentations of P. microphylla are conical in outline, with nearly 

 equal sides. 



Formation and locality: Cretaceous (Dakota group). Blackbird Hill, 

 Nebraska. 



