BUFFALO SOCIETY OF NATURAL SCIENCES 227 



2. Three genera, Stylonurus, Echinognathus, and Hughmilleria 

 were restricted to the rivers of Appalachia. 



A far greater interest must attach to the vast northeastern conti- 

 nent of Atlantica which stretched across the north Atlantic and formed 

 a land bridge of vital importance in the migration of the eurypterids. 

 The organisms living in the rivers of this continent were not geograph- 

 ically restricted like those in the rivers of Appalachia, whose remains 

 were washed out occasionally into the surrounding ocean waters, but 

 which were prevented from migration to European fresh waters by 

 the broad expanse of the Palaeozoic Atlantic; more fortunate by far 

 were the fluviatile inhabitants of Atlantica, for this continent, we may 

 feel sure, was fairly permanent throughout the Palaeozoic, even though 

 the ocean at times encroached over much of the southern part; it was 

 the northern portion that would be vital for the interlocking head- 

 waters of different river systems, and as we shall see there is over- 

 whelmingly convincing evidence pointing to such an intimate rela- 

 tion between the river systems of the periods from the Upper Siluric 

 through the Devonic. Not only were the geographical position and 

 extent of Atlantica more favorable for the widespread dispersion of 

 the eurypterids than were the same physical features of Appalachia, 

 but the sediments derived from the former continent were for the 

 most part of the particular lithological character most favorable to 

 the preservation of organic remains, while those from Appalachia were 

 quite often coarser, being prevailingly sandstones and conglomerates, 

 with only thin beds of intercalated muds. The early differentiation 

 in the character of the clastic deposits from these two continents re- 

 flects the still earlier difference which had existed between them in 

 the matter of elevation, for, whereas during Ordovicic and Lower Si- 

 luric (Niagaran) time the Canadian area, already peneplaned, had 

 been largely covered by the sea, as indicated by the remnants of Niaga- 

 ran limestones, and whereas during the same period the Baltic region 

 and that area now forming the southern shore of the Gulf of Finland 

 had likewise been covered by a shallow sea in which coral reefs flour- 

 ished, the continent of Appalachia on the contrary, had jutted up 

 from the Atlantic with lofty mountain ranges of crystalline rocks. 

 Thus it came about that the rivers in their slow but efficient work of 

 denudation brought into the waters bordering the continent of At- 

 lantica sediments that were calcareous and usually fine-grained 

 Cwaterlimes) while the rivers of Appalachia carried highly siliceous 

 materials of medium or coarse grain (sandstones and conglomerates) 

 and the winds transported siliceous sands. 



