22 Later Extinct' Flo North America^ 



This Bpeciea presents some marked characters by which it 

 iiiav be distinguished from those before described. It is true 

 that the variations of form among the leaves of onr recent 

 species of willow are almost infinite, and even in the same spe- 



. and from the Bame tree, leaves may be obtained of such 

 different aspect that taken separately they might readily be 



taken for those of different species. Since the difficulty 

 in the determination of recent willows is so great that it lias 

 ime proverbial, specific distinctions derived from the leaves 

 only, especially in those obtained from the same locality, may 

 justly be looked upon with suspicion. Here i where, 



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 out- 

 lines of theleaves can hardly be relied on. 



It will be seen by reference to the foregoing descriptions of 

 SaUces that a number of characters combine to distinguish 

 what, for geological convenience. I have chos» n to regard as dis- 

 tinct Bpecies. 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.flexuo saile, 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 Btrong, prolonged into a short robust petiole; 

 secondary nerves unequal, given off at a large angle, thick at 

 base, Blender, tortuous and irregularly confluent near the 

 margins. 



In 51 membranacea, the leaves are large and thin, broadest 

 near the base, which is rounded, summit long pointed and acute ; 

 ' i distinct and regular, but delicate. 



n and Locality. Cretaceous sandstone, mouth of 

 Si< -.\ !;'• ■ . Nebraska. 1 1 >r. Ha} den.) 



