DESCRIPTIONS OF SPECIES. 119 



These beautiful leaves are so definite in form and structure and so 

 perfectly preserved that we should have no difficulty in referring them to 

 their appropriate genus if we could find among living trees their precise 

 generic counterpart, but up to the present time I have not been able to 

 satisfy myself that they are generically related to any living plants. The 

 nervation is in some respects very like that of Berchemia, e. g., B. volubilis, 

 the "Supple Jack" of our Southern States. Nowhere else do I remember 

 to have seen the same parallelism of the secondary and Tertiary nerves, but 

 the serration of the margin is coarser than in any of the RhamnaceEe with 

 which I am acquainted, and the development of the basilar pair of lateral 

 nerves is much greater than in Berchemia. This latter character is not 

 without example in Rhamnus, as it is even more conspicuous in some species 

 of the genus, as, for example, in R. cettifolia of the Cape of Good Hope. 

 A cross between that species and our Berchemia, with a greater develop- 

 ment of the marginal dentation than either exhibits, would give us the 

 fossil before us. 



Considering it to exhibit more of the character of the Rhamnacea? than 

 of any other family, I have placed it doubtful^ there. 



Formation and locality: Tertiary (Fort Union group). Fort Union, 

 Dakota. 



Zizyphus hONGiFoLiA Newb. 



PL LXV, figs. 3-5. 

 Proc. U. S. Nat. Mus., Vol. V (March 21, 1883), p. 513. 



"Leaves 4 to 7 inches long by 6 to 12 lines wide; lanceolate, long- 

 pointed, wedge-shaped at base, and long petioled; margins waved, or more 

 or less distinctly toothed; midrib well defined from base to summit; basal 

 pair of lateral nerves approaching closely to the margin near the middle of 

 the leaf, then curving gently inward and anastomosing with the higher 

 lateral nerves, of which there are three or more set alternately and 

 curving upward, forming a festoon near the margin ; tertiary nerves very 

 finely reticulated." 



Of this species a large number of specimens occur in the Green River 

 Shales in certain layers where they are associated with the ferns Lygodium 

 and Acrostichum They may be at once distinguished from those of any 

 other described species of Zizyphus by their elongated and lanceolate form. 

 In the same slabs which contain these leaves are a few which, though 



