44 



BULLETIN 11, UNITED STATES NATIONAL MUSEUM, 



On the accompanying paleogeograpliic map (fig. 3) I have plotted 

 all of the locahties discussed in the preceding pages as well as other 

 areas where Black River and early Trenton strata are laiown to 

 contain the faunas under discussion. Except for local variations of 

 minor importance the distribution of these two formations in America 

 is essentially the same, so that this map will serve for both. Although 



Baltiymelrical Chart. 



Fig. 4.— Bathtmetric chakt of the North Polar Seas, suggesting routes of migration for mabine 



ORGANISMS. (After Nansen.) 



the Ordovician rocks of the Baltic area fail to outcrop some distance 

 east of St. Petersburg, due perhaps entirely to overlap by youngar 

 strata, they may extend across northern Russia under cover, for 

 outcrops are known in the Ural Mountains and along the shore of the 

 Kara Sea. The latter locahty is of particular interest in the present 

 connection, for here, along the shores of Yugor Strait, Nansen 



