60 THE FLORA OF THE DAKOTA GROUP. 



supra-basilar and opposite, ramose on the lower side, with nerville.s distinct 

 and at right angles. 



Third. By the denticiTlation of the borders, being much like that of the 

 species of Betxila or of Alnus, the teeth being more or less distinct, sometimes 

 marked by mere points on the border at the extremities of the secondaries 

 and of their divisions entering them. 



Fourth. By the long, slender petiole of the leaves. 



Of com-se there are some points of difference which may be remarked 

 in examining separately some of the leaves ; but none affecting them iu 

 their general, and therefore their generic, character. 



There is even between these leaves such a degree of affinity that their 

 separation into species is extremely diificult. They look as if they had 

 been derived from a single tree or from a group of low bushes of the same 

 species, of which, as is the case with plants of our time, some differences 

 are remarked in leaves separately examined and compared. The petiole of 

 these leaves bears at its base a small, bifid stipule, with lanceolate, pointed 

 lobes. This organism, however, is rarely preserved. 



The task of studying these leaves, repi'esented by hundreds of speci- 

 mens, has been hard, indeed, but at the same time pleasant and instructive. 

 For it has been rarely possible as yet to obtain in a fossil state such a 

 number of perfectly preserved fossil leaves, which, like these, may be 

 studied as easily as tliose of plants in the herbarium or even as if they were 

 still in process of growth, and thus afford clear evidence of the multiple 

 modifications which, in geological times, may have affected the plants of the 

 same species. It is admissible that if these leaves had been discovered 

 separately at divers times and at divers localities they might have been 

 refeiTed by authors to a nvimber of species. In the present case it is 

 scarcely possible to doubt their relation to a single species. Nevertheless, 

 in order to remark on some appreciable differences in their characters I 

 liave described them here under the name oi Betidites TFtA'/// and subdivided 

 the species into a number of varieties. 



Betulites Westii, sp. nov. 

 PL IV, Figs. 1-22 ; PI. V, Figs. 5-14. 



Leaves of small size, long-petioled, subcoriaceous, ovate-oblong, 

 obtuse or blunt at apex, truncate, subcordate or broadly wedge-form at the 

 entire base ; l)()rders sulientire or more or less distinctly denticulate ; ner- 

 vation pinnate, oraspedodrome ; primary nerves straight, narrow ; second- 



