STRATIGRAPHIC CLASSIFICATION PALEONTOLOGIC CRITERIA 487 



more of the basins in the Ohioan province. The Brassfield ("Ohio Clin- 

 ton" = earliest Clinton), the Brownsport, the Linden (Helderbergian), 

 the Chattanooga, the Osagian, and most of the Chester faunas likewise 

 invaded through the Mississippi embayment, but differ from the preced- 

 ing in passing westward by way of the Arkansas Yalle}^, into eastern and 

 sometimes to central Oklahoma. The southern Mesozoic and Tertiary in- 

 vasions also extend west of the Mississippi embayment, but these exten- 

 sions are limited to the flat southern border of the continent, including 

 the Great Plains, and did not pass between the Ouachita and Ozark areas. 



The Arctic center of dispersal. — The second important center of dis- 

 tribution of early and middle Paleozoic faunas apparently lay in tlu' 

 Arctic basin. From here certain stages of this slowly modifying fauna 

 invaded the Baltic region, the north Atlantic, northeastern and central 

 North America, northwestern Alaska, and the Cordilleran trough of west- 

 ern North America. While the shore life, more especially the gastropods. 

 pelecypods, and the brachiopods, on the American side is, as a rule, very 

 different in specific development from that prevailing on the European 

 side, there is yet a strong similarity in general aspect between the Cana- 

 dian, Ordovician, and Silurian faunas in the Baltic region and their near 

 contemporaries in certain parts of America. With respect to the middle 

 Silurian faunas this fact, as noted above, has been clearly established by 

 Weller. This similarity is shown particularly by types that either in 

 their early or in later stages of development are adapted to a floating 

 existence, and by others that frequently attach themselves to floating 

 objects — in other words, by species whose paths of migration are not con- 

 fined to shorelines. These readily migrating types consist chiefly of 

 corals, bryozoa, crinoids, cystids, and ostracods, but they include also 

 certain brachiopods, gastropods, and trilobites. Among the Silurian spe- 

 cies in northern Illinois, Iowa, and Wisconsin are some very extraordinary 

 genera of crinoids, corals, and trilobites whose foreign alliances are at 

 once apparent. 



The migration of these Silurian genera between the Chicago and Baltic 

 regions probably occurred along the northern shore of the land believed 

 to have connected northwestern Europe with Newfoundland and Labra- 

 dor during most of Paleozoic time. Southeast of this land, in Europe, 

 is the Baltic trough or basin which received hiost of its Paleozoic faunas 

 from the Arctic. At times the Baltic trough connected the Arctic and 

 the middle Atlantic basins, permitting, as stated in a preceding para- 

 graph, migrations of Baltic types in a southwestern direction as far as 

 the Mississippi embayment. These migrations, like many preceding, and 

 probably others following them, are supposed to have taken place along 

 the southern shore of the hypothetical land connection. 



