24 GIRTY 



upper Mississippian, and which without much doubt belongs in 

 the Western Upper Carboniferous in the horizon of the Hue- 

 conian series (Weber quartzite). Furthermore, being called 

 upon nearly 2 j^ears ago to determine the boundary between the 

 Upper and Lower Carboniferous in northern Arkansas, a careful 

 faunal study led me, as already described, to draw the line at 

 the top of the Archimedes limestone. Thus the genus Pentre- 

 mites, which, equally with Archimedes, has been supposed to 

 be infallibly diagnostic of Mississippian time, is found to occur 

 in abundance in the Upper Carboniferous (Pentremital lime- 

 stone ; sparingly in the Kessler), and although Archimedes 

 practically dies out in this area with the Archimedes limestone, 

 a few fragments representing it occur in the beds above. It 

 appears, therefore, that even in the Mississippi Valley this strik- 

 ing genus ranges above the top of the Lower Carboniferous, and 

 while only a very scanty representation has thus far been found, 

 unless the Archimedes limestone proves to belong in the Potts- 

 ville rather than in the Mississippian, there is no reason to be- 

 lieve that it did not survive in abundance in other regions, as, 

 indeed, proves to be the case. 



Tschernyschew also correlates the Russian section with that 

 of the Mississippi Valley. His correlation may be correct, but 

 the Pennsylvanian faunas of the latter area are so widely dif- 

 ferent from those of our Western States which the Russian 

 ones most closely resemble, that, in the opinion of one who has 

 some acquaintance with both types, a precise correlation is, in 

 our present knowledge, impossible. The beds placed in align- 

 ment by Tschernyschew contain faunas so widely dissiiT\ilar that 

 it seems an act of temerity to group them together. The evidence 

 for so doing consists in part of the occurrence of certain Ameri- 

 can species in the Russian faunas, but the identifications, if one 

 may judge by the tigures given, in some cases are questionable 

 and in others consist of such long-ranged types that in view of 

 the reall}' small percentage which these forms bear to the en- 

 tire fauna, the evidence appears of diminishing significance 

 the more critically it is examined. 



I should not be understood, however, as expressing a belief 

 that these Western faunas do not in a general way, in part at 



