170 UNITED STATES GEOLOGICAL SDRVEY— TERTIAEY FLORA. 



the teeth or divisions of the borders, or curving along them. The areas, 

 divided in right angle by fibrillae, form primary irregularly square meshes, 

 and the veinlets branching in various directions ultimately constitute a very 

 small polygonal areolation. One pair of slender marginal nerves pass from 

 the top of the petiole below the primary lateral veins, joining their branches 

 by veinlets. These general characters are, however, modified in many 

 ways The nervation, especially, is very variable and complex, for, by the 

 addition of one or two jiairs of nerves under the primary one, it becomes 

 five- or seven-palmate, while in other leaves, as for example in those of Popu- 

 Im hdlsaynifera var. nngusiifolia, a species especially common in the valleys 

 and along the base of the Rocky Mountains, the lower primary nerves are 

 alternate, of the same thickness as the secondary ones, all equidistant and 

 equally branching underneath, representing thus a pinnate nervation, rendered 

 remarkably similar to that of the leaves of Salix by the interposition of short 

 tertiary veins, traversing the lower side of the areas and dissolving in their 

 middle by subdivision in nervilles. In other cases, as in the so multiform 

 leaves of Populus alba, an introduced species, too common in this country, 

 the primary lateral nerves, two or three pairs, stronger and more branching 

 than the secondary ones, go straight to the points of acute lobes or of larger 

 teeth ibrmed by expansion of the laminse, are thus craspedodrome, as well as 

 some of the secondary ones, and give to the leaves some of the characters 

 and the appearance of leaves o^ Platanus. 



These variations in the characters of the leaves, even of the same species, 

 render their determination very difficult and somewhat unreliable ; for the 

 paleontologist has rarely for examination and study a series of specimens so 

 numerous, and in such a perfect state of preservation, as are those which 

 have served for the preparation of the admirable monograph of this genus 

 in Heer's Fl. Tert. Helv., where are represented branches bearing leaves 

 of various forms, some entire, some dentate, besides buds, bracts, catkins of 

 flowers, and seeds. 



The difficulty of determination of the leaves of Populus may, in a certain 

 degree at least, account for the great diiference between the number of spe- 

 cies of this genus known as living at our time, eighteen only,* and the fossil 

 ones, of which Schimper's Pal. V^gdt. describes sixty-two, considering, how- 

 ever, nineteen of thcni as (h)ubtful Of the species of Poplars living at our 



* According to the monograph of Wesniael, iu De Candolle's ProdrouiuB. 



