[PENHALLOw] KOTES ON TERTIARY PLANTS 49 



critical compaarison with this last species, especially from the stand- 

 point of the longitudinal sections, sihows a want of agreement in some 

 very important respects. The accompanying list of dimensions may 

 be introduced here to show in part, the resemblance to Populus bal- 

 samifera. While such data have no absolute value for diagnostic pur- 

 poses, they are nevertheless useful as supplementing other data, and 

 will be referred to later. 



Comparison of Cells and Cell Walls. 



Size of Cells. Thickness of walls. 



Spring wood. Spring wood. 



Rhamnacinium porcupinianum 20.0 ^ 9.37// 



Populus balsamifera 21.3 fi 9.Z7/li 



Populus fremonti 22.5 u 6.25 /^ 



Salix longifolia 13.7 fi 6.25// 



Salix bigelovii 14.5 fi 6.25// 



Rhamnus caroliniana 15.3 ju. 4.7 ju 



Rhamnus purshiana 14.2 ;/ 3.1 // 



In 1896 Felix described a wood from the Yellowstone National 

 Park to which he applied the name of Rhamnacinium radiatum.^ As 

 figured and described, this wood bears a remarkable resemblance to 

 our specimen which is again comparable with another wood from the 

 Yellowstone National Park described by Knowlton in 1899,- and referred 

 by him to Ehamnacinium radiatum, though regarded as such with some 

 hesitation, on account of its striking resemblance to a poplar. It 

 would thus appear that there is considerable doubt attached to the 

 identity of this fossil, which it is important to remove; and the diffi- 

 culty is greatly increased by the very striking resemblance which is to 

 be found between the structural details of the wood of the Rhamnacege 

 and the Salicacege. It, therefore, becomes necessary to detennine 

 1st. If our specimen belongs to the Khamnaceœ or the Salicacese. 

 2nd. If to the latter, w^hether it is a Salix or a Poplar. 

 3rd. If to the former, whether it is identical with the Rhamnacinium 

 ■of Felix, or with the somewhat doubtful one of Knowlton, 

 or with both. 



To the solution of these questions I shall endeavour to bring evi- 

 dence derived from existing species of both families, in the hope that 

 it may furnish a final answer. 



The state of preservation of the specimen is such that considerable 

 difficulty has been experienced in obtaining a diagnosis which I could 



^ Zeitschr. d. Deutsch. geol. Gesell., 1896, 252, 

 ^ Flora of the Yellowstone Nat. Park, 769. 



Sec. IV., 190;^. 4. 



