56 THE LATER EXTINCT FLORAS OF NORTH AMERICA. 



of such different aspect that, taken separately, they might readily be mis- 

 taken for those of different species. Since the difficulty in the determina- 

 tion of recent willows is so great that it has become proverbial, specific 

 distinctions derived from the leaves only, especially in those obtained from 

 the same locality, may justly be looked upon with suspicion. Here, as 

 elsewhere, however, it is probable that recent botany will derive some aid 

 from the careful study of fossil plants, and the nervation will probably 

 be found to afford constant characters where the outlines of the leaves can 

 hardly be relied on. 



It may be seen by reference to the foregoing descriptions of Salices 

 that a number of characters combine to distinguish what, for geological 

 convenience, I have chosen to regard as distinct species. 



Salix Meekii is lanceolate, tapering nearly equally to both ends, which 

 are alike acute; this leaf is petioled and the nervation regular and delicate. 



S. flexuosa is sessile, linear, and rather abruptly narrowed to point and 

 base; nervation obscure, apparently very delicate and uniform. 



S. cuneata is comparatively thick and leathery, the form symmetrical, 

 lanceolate, pointed, but scarcely acute at both ends; the midrib strong, 

 prolonged into a short, robust petiole; secondary nerves unequal, given off 

 at a large angle, thick at base, slender, tortuous, and irregularly confluent 

 near the margins. 



In 8. menibranacea the leaves are large and thin, broadest near the 

 base, which is rounded, summit long-pointed and acute ; nervation distinct 

 and regular, but delicate. 



Formation and locality: Cretaceous (Dakota group). Mouth of Big Sioux 

 River, Nebraska. 



Salix flexuosa Newb. 



PI. II, fig. 4; XIII, figs 3, 4; XIV, fig. 1. 



Ann. N. Y. Lye. Nat. Hist., Vol. IX (April, 1868), p. 21 ; Ills. Cret. and Tert. PL 

 (1878), PI. I, fig. 4. 



" Leaves narrow, linear, pointed at each end, sessile or very short- 

 petioled; medial nerve strong, generally somewhat flexuous; secondary 

 nerves pinnate, leaving the principal nerve at an angle of about 40 degrees, 

 somewhat branched and flexuous, but arching so as to inosculate near the 



