488 E. O. ULRICTI REVISION OF THE PALEOZOIC SYSTEMS 



The Ordovician faunas of the Eussian Baltic area so far described have 

 seemed to give little promise for exact correlation with interior American 

 deposits. A few of the trilobites, gastropods, and brachiopods suggest 

 species found in the upper Mississippi Valley, but taken as a whole the 

 Eussian faunas of these classes are strangers to the American paleontolo- 

 gist. However, studies of the Eussian Ordovician bryozoa now being 

 carried on by Dr. E. S. Bassler are bringing out some surprising and 

 highly interesting results. So far about 70 out of 143 species described 

 from the Decorah shales and the lower members ("Clitambonites^' and 

 "Nematopora'^ beds) of the Prosser limestone in Minnesota have been 

 identified in the Jewe and Wesenberg formations of Eussia. Also several 

 Decorah shale ostracods are represented in these Eussian formations by 

 closely allied or identical species. It appears, further, that some of the 

 gastropods in the lower part of the Lyckholm may be indistinguishable 

 from species characterizing the top beds ("Fusispira bed") of the Prosser 

 limestone in Minnesota. Finally, American Eichmondian faunas are 

 clearly recognized in the upper Lyckholm and the Borkholm.^® 



In discussing the effect of marine currents on the distribution of 

 faunas in continental seas in an earlier part of this paper (pages 365-871) 

 the great extent of some of these Ordovician Arctic invasions of north- 

 eastern America is described in sufficient detail. So far as known, the 

 first of these broad invasions occurred in the Black Eiver epoch. The 

 last to reach the United States of those coming in by way of Hudson 

 Bay appears to have been the late middle Silurian Guelph fauna. All of 

 them followed essentially the same pathways and covered in part or whole 

 the same areas. The subsequent middle and late Devonian and Waver- 

 lyan invasions by Arctic faunas came in by the northwestern route, and 

 differed thus from the extensive Ordovician and Silurian invasions. 



According to evidence now at hand, it is thought highly probable that 

 migration occurred also along the north Eussian and Siberian shores be- 

 tween the Baltic region and Alaska. Whether the Paleozoic Arctic migra- 

 tions were ever quite circumpolar, with free interchange between Alaska 

 and Baffinland, is somewhat doubtful. Certain it is that, so far as known, 

 the Niagaran faunas in Alaska are much less like those in eastern Canada 

 than they are like those in west central and northeastern Europe. How- 

 ever, the Ordovician and Eichmondian faunas in Alaska are not very 



5» In connection with tlie last fact there is to be noted a very interesting coincidence, 

 namely, that in Russia, as in the Mississippi Valley and in the northern and western 

 parts of North America generally, the beds containing the Richmond fauna rest directly 

 on beds holding a rather early Trenton fauna. Where these stratigraphic relations 

 occur in America a well defined unconformity separates the two faunas. Probably a 

 similar unconformity occurs within the Lyckholm. 



