PALEOBOTAXICAL EVIDEXCE 511 



"Either on account of laud or marine barriers, or because tlie climatic con- 

 ditions throughout the Northern Hemisphere may at the outset have been less 

 uniform than in the preceding epoch, the different areas exhibit more or less 

 distinct local floral differences. Thus in the Pocono of West Virginia and 

 eastern Pennsjlvania, where Triphyllopteris and the corrugatum type of Lepi- 

 dodendron are almost without competition, the former achieved a remarkable 

 differentiation far surpassing that known in any other area. In Nova Scotia, 

 on the other hand, the Horton series, which I regard as practically contem- 

 poraneous with the Pocono, contains the same Lepidodendra, accompanied, 

 however, by Aneimites instead of Triphyllopteris. In both the regions the 

 formations are in close relations with the Upper Devonian. . . . All the 

 genera mingle in Arctic Europe and Siberia, where Cyclostigma, probably of 

 Arctic birth, has a good development." 



In middle Mississippian time the earlier flora has been largely re- 

 placed b}' one that is very different in aspect. Here, says White, '^'^^'e 

 find a flora essentially consisting of Ehacopteris, Cardiopteris, Astero- 

 calamites (=Bornia), with Lepidodendron volkmannianum, and L. 

 veUlicimianuiii, accompanied by a gradually increasing group of Sphenop- 

 terids." 



This flora has a very wide range in the Old World. Originating or 

 best known in western Europe, it extended westward into Siberia, and 

 southeast probably through the Balkans and Persia to South Africa and 

 Australia, and thence presumably by way of ^^Gondwana land" to Argen- 

 tina, in the New World. The flora of this time in the Appalachian region 

 is so little known that close comparisons are not yet possible. 



The climate during middle Mississippian time was undoubtedly mod- 

 erate and uniform. Eor although the members of this flora "do not 

 attain the gigantic proportions nor the specific differentiation of their 

 Carboniferous successors, yet the relative homogeneity and the great 

 radial distribution of this flora argue for the absence of distinct climatic 

 zones in the recent sense, while the apparent lack of annual rings, so far 

 as the woods have been specially examined, is opposed to the idea of 

 seasonal changes.-'^ 



Knowledge of the upper Mississippian flora is still far from satis- 

 factory, either as to composition or geographic range; hence only tenta- 

 tive conclusions can be drawn as to the probable climatic conditions 

 under wdiich it lived. From the fact that certain of the plant types 

 exhibit somewhat depauperate, semi-coriaceous foliage, and the possible 

 presence of growth rings in certain of the woods. White considers it as 

 at least "provisionally admissible'^ that there were somewhat severer 

 climatic conditions with more or less seasonal change. 



Pennsylvania floras. — With the inauguration of Pennsylvania time 



