^ 



226 



BULLETIN OF THE 



It is not less valuable in regard to the data which it affords in con- 

 firmation or contradiction of Komc of the general conclusions derived 

 from the examination of the materials formerly described from this 



F 



peculiar cretaceous flora. 



For example, the disconnection of the flora of the Dakota group from 

 that of the older ones, those of the Jurassic times, does not appear now 

 as positive as formerly, or as it was indicated in tiie Cretaceous Flora, 

 Vol. VI. of the U. S. lleports. For besides the Pterophylkim described 

 in that volume as somewhat doubtfully refei^able to the Cycadcix;, plants 

 which essentially constitute the vegetation of the Jurassic, we have now 

 four species of the genus Podozamites either identical with or closely 

 related to species known from this formation. 



Per contra^ the disconnection of the Cretaceous flora from that of the 

 lower Tertiary appears now still more evident, as the new species do 

 not indicate any afiinity to the plants of the Laramie group, which is 

 positively Eocene by its types. As yet no vegetable remains distinctly 

 referable to Palms have been found in the Dakota group. 



The vegetation of the Cretaceous seems to be essentially composed of 

 synthetic types, genera or groups of plants of analogous characters, 

 which it is extremely difiicult to define and separate into species. And 

 nevertheless, when considered separately, the leaves, which are as yet the 

 only organs obtained for analysis, show points of diftcrence so marked 

 that it is not well possible to consider them as representing mere varieties. 

 To prove this assertion, it is sufiicient to examine the so-called species 

 described imdcr the generic names of PopuUtes, Platamis, Araliopds (a 

 new division which includes most of the leaves described as Sassafras), 

 Aralia, Protcqihylluin, Menisperiniies, etc. One of tliese types, however, 

 the more peculiar and distinctly cretaceous in its facies, Liriodendron, 

 was until now represented by few diversified forms, and therefore con- 

 sidered as simple, and the supposition seemed confirmed by its passage, 

 with Platama, Sassafras, and Far/us, through all the geological ages from 

 the Cretaceous to our time, with scarcely any modifications and few 

 representatives. But now (he collection of Mr. Sternberg eliminates 

 this exception, as we find in it four new and very distinct forms of tliis 

 rronp, besides a beautiful entirely preserved leaf of Liriodendron (jiyan^ 

 teum, which had been described from a mere lateral lobe, and which w^as 

 therefore of uncertain attribution. 



A short descripti(m of this genus, and of its species as ex])oscd by the 

 leaves, may show how uncertain, though binding, are the characters of 

 the Cretaceous leaves. 





