496 Evolution and Distribution of Fishes 



back the continuity of Dipneusti to Lower Old Red beds. 

 Curiously enough also, as with the most ancient Cross- 

 opterygli, the area for evolution of, and early distribution 

 of, these four genera stretched from Canada and eastern 

 U. S. through Britain and France into W. Russia, though 

 a region that centred round east and north-east Scotland 

 may have been the most active and ancient home for the 

 evolving genera and sub-families. 



Accepting the views of Newberry, Woodward, and 

 Eastman as to the close affinities of primitive Dipneusteans 

 with the Arthrodira, the latter lived through a much short- 

 er period, and inhabited a greatly more restricted territory 

 than did the former. This is almost certainly due to their 

 increasingly heavy' armament, also to their having adopted 

 an even more pronounced method of ground-feeding than 

 did the Dipneusti. From the time of their undoubted first 

 appearance in Lower Old Red beds, till their extinction in 

 upper beds of the same formation, the freshwater area 

 occupied extended from at least Ohio and Illinois eastward 

 through Britain to the Orkneys, and thence northward to 

 Spitzbergen — where Lophostracon spitzbergense has been 

 gathered — to N. W. Russia, also eastward through Belgium 

 to the Eifel region of Germany. 



Such distribution would suggest a rather more expanded 

 N. Atlantis continent than that outlined by Freeh {105). 

 If Barrande's fragmentary specimens of a supposed Coc- 

 costeiis from the Silurian of Bohemia are correctly identified 

 (506:638-40) the group may have originated in some 

 shallow lakes of Central Europe, from which new and 

 larger types may have radiated, till all culminated in the 

 huge and cumbrous Homosteus, Dinichthys, and Titanich- 

 thys of Russian, Scotch and eastern N. American territory. 

 So while the Dipnoi evolved along structural lines that 

 were adaptive and capable of receiving Impressions from 

 without which could be fairly rapidly linked up Into a pro- 

 environal response, the Arthrodira, showing such capacity 

 in less marked degree, survived only through the Old Red 

 period, and became extinct in the early Carboniferous. 



The three ancient divisions of the Antiarchi, the Osteo- 

 straci and OstracodermI, date back from Upper Devonian 



