48() E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



along the eastern shore of America to the Gulf of Mexico and on through 

 the Mississippi embayment to western Tennessee and northern Arkansas. 

 This migration is indicated by the Saint Clair limestone of northern 

 Arkansas and eastern Oklahoma — a pure crystalline limestone formation 

 corresponding in age to some part of the Clinton of the Appalachian Y al- 

 ley and central 'New York — in which we find a number of species that are 

 closely allied to Baltic, British, and Bohemian types. Also in the later 

 Brownsport formation of western Tennessee there are many other species 

 having similar relations. Weller, who has given much thought to the 

 Silurian migrations, assumes that the European genera and species in 

 western Tennessee and northern Arkansas reached these localities by 

 simple southward extension of the Arctic invasion.^ ^ At present I can 

 not accept this view, being convinced that there is no direct stratigraphic 

 connection between the pure limestones of the Saint Clair and Browns- 

 port formations in the south and the dolomitic Silurian limestones in the 

 north. Besides, most of the European genera of crinoids in the southern 

 formations mentioned are different, or are represented by distinct species, 

 from those found in northern Illinois and Wisconsin, which would 

 scarcely be so if they had migrated southwardly across the continent. 

 The distribution of Scyphocrinus, which, with its peculiar floating bulb 

 {Camarocrinus) , is found in late Silurian or early Devonian rocks in 

 Tennessee, Maryland, Great Britain, and Bohemia, but neither in the 

 Baltic region nor in the Silurian dolomites of North America, is very 

 significant in this connection, since it proves that the Atlantic-Gulf of 

 Mexico route was in use during at least a part of Silurian time. 



Gulf of Mexico invasions. — Concerning the periodic invasions of North 

 America by the Gulf of Mexico fauna, sufficient positive knowledge has 

 accumulated to justify a number of fairly detailed inferences. As a rule 

 this fauna entered the continental area through the Mississippi embay- 

 ment, the probable structural connection between southern Appalachia 

 and Llano apparently being frequently submerged. Often the fauna did 

 not spread west of Ozarkia, being confined in these cases to the Ohioan 

 province,^^ a subtriangular region bounded by irregular lines connecting 

 Minnesota, Quebec, and the mouth of the Mississippi River. The lower 

 and upper Stones Eiver, the Lowville, the middle and later Trenton, the 

 middle and later Cincinnatian, the Arnheim Richmondian, the Rochester, 

 the Waldron, the Onondaga, the Hamilton, the Glen Park Kinderhookian, 

 the Spergen, the Saint Louis, and apparently the Saint Genevieve, all of 

 these faunas invaded from the Gulf of Mexico and are confined to one or 



^ Stuart Weller : Journal Geology, vol. vi, pp. 692-703. 



« U. S. Geological Survey, Prof. Paper No. 24, 1904, note p. 91. 



