144 DAVID WHITE 



Alaskan where the same hnear-lobed Sphenopteris forms are also 

 present; the Nova Scotian affiliates more closely with the eastern. 

 All the genera mingle in Arctic Europe and in Siberia, where Cyclo- 

 stigma, probably of Arctic birth, has a good development. The 

 Pocono flora may have connected by a more northern route with 

 Europe, where Triphyllopteris sparingly mingles with the Horton 

 Aneimites, which presumably migrated by the Northeast Arctic land 

 bridge. It is possible, however, that some of the regional differences, 

 especially differences in species, are due to lack of exact synchronism 

 in the plant beds of these remote regions. 



Middle Mississip plan flora. — The basal Carboniferous floras are 

 largely replaced in the middle Mississippian by a plant association 

 which is more varied and of very different aspect. Where conditions 

 were favorable for plant growth and preservation we find a flora 

 essentially consisting of Rhacopteris (of Schimper, including Rhodea 

 of authors), Cardiopteris, Asterocalamites ( = Bornia), with Lepi- 

 dodendron volkmannianum, and L. veltheimianum, accompanied by a 

 gradually increasing group of Sphenopterids. 



Source and distribution. — The middle Mississippian flora probably 

 had its greatest development among the islands and estuaries of 

 western Europe; at least it is best known in that region. From there 

 it seems to have extended almost homogeneously to the eastward into 

 Siberia and to the southeast, either through the Balkans, Persia, and 

 the Himalayas (linking together its discovered occurrences), or pos- 

 sibly by a more southern route, to South Africa and Australia where 

 the flora was largely identical with that in Siberia. The flora at 

 Cacheuta, in Argentina, which though small is mainly composed of 

 European species, presumably traversed the same route as that later 

 followed by the Gangamopteris flora — that is the "Gondwana land." 

 The middle Mississippian flora of the Appalachian trough is but little 

 known, and for the most part is unpublished. Though less closely 

 bound to that of Europe than are the corresponding Pennsylvanian 

 floras its genera and a number of its species are present in the basins 

 of Europe. I may add that the Megalopteris-bearing beds along the 

 Mississippi River in Illinois, long ago credited to the Chester, are of 

 upper PottsviUe age. 



Moderate uniformity of climate. — The members of this flora do not 



