2l6 THE HABITAT OF THE EURYPTERIDA 



Theory of River Habitat. To pursue this marine-lagoon the- 

 ory to its logical conclusions in every case would use up many pages 

 of print and would always lead to absurdities, impossibilities or con- 

 tradictions. Therefore, without dwelling longer on the perplexities 

 and inconsistencies attendant upon this theory, I shall pass at once 

 to the development and exposition of the theory of river habitat. 

 Throughout the Palaeozoic there were in existence in the northern 

 part of the western hemisphere three continents which, though vary- 

 ing much in size from period to period, often becoming confluent and 

 at times even being largely covered by the epicontinental sea, never- 

 theless preserved a marked degree of integrity. These three conti- 

 nents were (i) Appalachia, which occupied what is now the eastern 

 border of North America, and constituted the northward projection 

 of the land mass now known as South America, and which supplied 

 the greater part of the clastic materials deposited in eastern North 

 America throughout the Palaeozoic; (2) Rockymontana, which lay for 

 the greater part of its length on the present continental mass extend- 

 ing from Mexico to Alaska, a palaeocordilleran chain, from which clas- 

 tic sediments were derived which were deposited on the western bor- 

 der of North America and along the continental shelf; (3) Atlantica, 

 the great northern North American and northwestern European con- 

 tinent one portion of which, the Canadian shield, was formerly sup- 

 posed to have been the source of nearly all of the Palaeozoic clastic 

 deposits over what is now the United States. Throughout the 

 Palaeozoic this Canadian area was usually connected with the Scot- 

 tish and Scandinavian masses by a broad strip of land extending 

 across the North Atlantic (see map, fig. 8). There was a fourth and 

 smaller continent, Mississippi*!, occupying the area of the Missis- 

 sippi valley, which at times was entirely covered by the sea, and again 

 formed a part of Rockymontana. Each of these continents had its 

 own river systems, the organisms living therein being subject to the 

 laws of migration and dispersal which are seen to be operative now. 

 Furthermore, the fluviatile fauna of each continent would be dis- 

 tinct as a rule. If, however, migration in circumpolar belts occurred 

 and fluviatile organisms from one continent passed to another, these 

 migrant forms would yet show their closest affinity not to species liv- 

 ing in the rivers of the continent to which they were immigrant, but to 

 those in the rivers of the continent from which they emigrated. In 

 any given period faunas v hich can be shown to have come from rivers 

 on the same continent should be more closely related than faunas 



