Theory of River Habitat. To pursue this marine-lagoon theory to its logical conclusions in every case would use up many pages of print and would always lead to absurdities, impossibilities or contradictions. 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 Palæozoic there were in existence in the northern part of the western hemisphere three continents which, though varying much in size from period to period, often becoming confluent and at times even being largely covered by the epicontinental sea, nevertheless preserved a marked degree of integrity. These three continents were (1) 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 Palæozoic; (2) Rockymontana, which lay for the greater part of its length on the present continental mass extending from Mexico to Alaska, a palæocordilleran chain, from which clastic sediments were derived which were deposited on the western border of North America and along the continental shelf; (3) Atlantica, the great northern North American and northwestern European continent one portion of which, the Canadian shield, was formerly supposed to have been the source of nearly all of the Palæozoic clastic deposits over what is now the United States. Throughout the Palæozoic this Canadian area was usually connected with the Scottish 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, Mississippia, occupying the area of the Mississippi 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 fiuviatile fauna of each continent would be distinct 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 living 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 which can be shown to have come from rivers on the same continent should be more closely related than faunas
