EAELY PALEOZOIC BEYOZOA OP THE BALTIC PROVINCES. 45 



collected a fauna which Kiaer ^ correlated with the Baltic Cl. With 

 such an eastward extension of the Baltic Ordovician faunas, it is 

 quite possible that in time they may be proven to be circum-polar. 

 East of the Ural Mountains and west of Alaska little of the Ordovician 

 geology is known, so that here, in drawing the outlines of the Black 

 River-Early Trenton Sea, I have simply followed the present day 

 coast. 



In this connection Nansen's bathymetric chart of the North Polar 

 Seas, wliich I have introduced (fig. 4), is of interest in showing possi- 

 ble routes of migration for marine faunas. Believing in the perma- 

 nence of continental land masses and oceanic deeps, it is thought that 

 essentially similar conditions prevailed in early Paleozoic times. 



THE BRYOZOAN FAUNAS. 



Although the bryozoans in many instances constitute the larger 

 portion of a fauna, the study given them is almost invariably less 

 than that of any other class. This is due to the fact that they have a 

 reputation for being one of the most difficult classes of organisms. 

 Their small size requires the use of a hand lens or a microscope, and 

 in the fossil forms frequently the preparation of tliin sections. Yet 

 the necessity for tliin sections is to the advantage of the student, for 

 by their use he is less liable to error. It is in tliis very fact that the 

 fossil Bryozoa are so valuable to the stratigrapher. In areas where 

 other fossils are too imperfect or ill preserved for identification, the 

 merest fragjnents of bryozoans can be identified with certainty by 

 properly prepared thin sections. Again, the bryozoans are of great 

 importance to the stratigraphical geologist because they occur 

 usually in great abundance over wide areas and preserve their specific 

 characters with little change. 



Of the various classes that are represented in the Ordovician rocks 

 of Baltic Russia, bryozoans in all probability rank first in number of 

 species and individuals. As indicated on some of the plates at the 

 end of this volume, certain shales are often crowded with them and 

 thin limestone bands are frequently one mass of their fragmentary 

 remains. 



All of the trepostomatous bryozoans and many of the Cryptos- 

 tomata and Cyclostomata require thin sections for their initial study. 

 Directions for the preparation of such thin sections have been given 

 in the report of the "Geological Survey of Illinois," volume 8, 1890, 

 page 292 ; in the ' Final Report of the Geological and Natural History 

 Survey of Minnesota," volume 3, 1893, page 100; in "Bulletin 173 of 

 the United States Geological Survey," 1900, page 122, and, more 

 recentl}^, in the "Thirty-second Annual Report of the Indiana 



1 J. Kiaer in Nansen's North Polar Expedition, vol. 4, No. 11, 1902. 



