20 CRETACEOUS AND TERTIARY FLORA. 



specific forms have been described from Nebraska and Kansas — some of 

 them extremely well defined. This shows, perhaps, more evidently than 

 any other fact remarked on the characters of the plants of the Dakota 

 Group the great disposition to variableness by modification of some charac- 

 ters in the first Dicotyledonous plants. These changes have either caused 

 a multiplication of specific forms preserving traces of the original types 

 in traversing the subsequent geological formations, or have gradually 

 destroyed the number of specific representatives of some genera, as in 

 Liriodendron, or even caused the total disappearance of some of the best 

 defined and more predominant types, like those of Oredneria, Pterophyllum, 

 etc. Of these, however, the original characters may have been so widely 

 varied that the ultimate derived forms have not yet been distinctly recog- 

 nized on plants living now. The two last-named genera, Oredneria and 

 Protophyllum, may possibly be referable to some subdivisions of the Colum- 

 niferce, the Buttneriaceoz and Pterospermce, for example. 



The three species which I have described under the insufficiently- 

 defined genus of Sterculia are all very uncertain in their relation. As 

 much may be said for the following and last classes of the vegetable 

 kingdom: 



To the Acerece is referable Negundoides acutifolius. The leaf, however, 

 as seen from pi. xxi, fig. 5, and its description, is too fragmentary for a 

 satisfactory determination of its characters. Acer antiquum is described 

 by Ettingshausen in his "Flora of Niedershcena," but from the opinion 

 of the author the reference is uncertain. The leaf rather resembles a 

 deformed form of Quercus or of Liriodendron. In the same order Heer has, 

 from the Upper Cretaceous of Greenland, a Sapindus prodromus, repre- 

 sented by one leaf only, which has evidently the character of the genus. 

 A beautiful species of Sapindus described here from Colorado is also present 

 at Atane. This genus is therefore Cretaceous. The reference to the Rham- 

 nacece of the leaf described as Rhamnus tenax in "Cret. Fl." is apparently 

 legitimate, for of the same group three other species, P. prunifolius, a 

 Celastraphyllum, and an Ilex, are described here from the same formation. 



To the Anacardiacece we have probably to refer, as Rhus Bebeyana, the 

 species described as Populus and as Juglans Debeyana as seen in "Cret. 

 Fl., p. 110. I have not obtained from the Dakota Group any new materials 



