EAELY PALEOZOIC BEYOZOA OP THE BALTIC PROVIjSTCES. 41 



classes of organisins which require a shore line for migratioii have 

 been most studied in the two areas and sufhcient resemblances 

 between them have not been noted. The types which are less de- 

 pendent on continuity of shore line are the corals, crinoids, ostracods, 

 and bryozoans, and these show the greatest similarity in the two 

 areas. The trilobites, gastropods, and other classes usually requiring 

 such a shore line only occasionally exhibit identical species. Some 

 of their species suggest forms of the Mississippi Valley, but as a rule 

 these faunas are strange to the American paleontologist and would 

 serve very poorly for exact correlation with the American strata. 

 However, the classes to which a shore line is not essential for migra- 

 tion show numerous species in common. Thus, the br3^ozoans, which 

 m the Baltic area have received comparatively little attention, show 

 as a result of the present study that 65 species out of 161 are common 

 to this region and to America. 



In correlating the Russian strata, hitherto referred to the Ordo- 

 vician, with only middle Ordovician and early Silurian strata of 

 America, other factors than faunal similarity must be considered. 

 According to the correlation table on page 38 all strata of Chazyan, 

 upper Trenton, and of Cmcinnatian time are unrepresented in the 

 Baltic section if the presence of numerous identical species is to be 

 counted a good criterion in correlation. Excepting the long-lived 

 Corynotrypa delicatula and a few other cyclostomatous bryozoans, so 

 far as America is concerned, not a single species of the numerous 

 Mohawkian bryozoans is known in the underlying formations. Simi- 

 larly, the upper Trenton and Cincinnatian strata contain no species 

 of bryozoans in common with the underlying early Trenton and older 

 rocks. With the large number of species that has been described, 

 this is most conclusive evidence for the distmctness of the faunas 

 and upon these data alone the correlation of the Russian strata here 

 advocated would be justified. 



The Black River strata containing the greatest percentage of Rus- 

 sian species are thickest in the northern part of North America and 

 overlap to extinction south v^^ard. For example, the southernmost 

 outcrops of the Decorah shale are in northern Tennessee where they 

 occur very locally overlying the Lowville limestone and are here 

 never over a few feet in thickness. Similarly, the lower Trenton 

 horizon designated as the Clitambonites and Nematopora beds in my 

 section on a previous page, overlap southward until in southern Ten- 

 nessee, where they rest upon the Lowville limestone, they are only 

 a few inches in thicloiess. Besides this, there is dias trophic evidence 

 that the seas of this time entered the continent from the north. 

 In Russia, on the other hand, we have no stratigraphical evidence 

 of corresponding overlaps of Ordovician strata upon any rocks 

 younger than the Dictyonema shales. 



