ENVIRONMENTAL CONDITIONS 



403 



deposits, and only there, at the delta margin, were found the conditions 

 favorable for preservation. 



Broad tracts in the central parts of the continent were, however, cov- 

 ered with the marine waters of epeiric seas, and in the deposits of these 

 are preserved the remains of marine fishes. In the Lower Devonian the 

 fossils come only from the Maine-New Brunswick province. Here delta 

 and bay conditions prevailed, and the fauna can not be regarded as truly 

 marine. In the following tabulation of the truly marine Devonian fishes 

 of North America all from the Lower Devonian were accordingly ex- 

 cluded. All from the Middle Devonian were included, but of the Upper 

 Devonian only the fossils from Ohio, Kentucky, Iowa, and Illinois were 

 included. 



Marine Devonian Fishes of North America 





Middle Devonian 



Upper Devonian 



Subclasses 



Ulsterian 



Erian 



Ohio, Kentucky, 

 Iowa, Illinois 





Genera 



Species 



Genera 



Species 



Genera 



Species 



Selachians 



Shagreen 

 8 

 4 



1 

 



granules 

 11 

 6 



9 

 4 



11 

 8 

 2 



2 



12 



12 



3 



1 



28 

 27 



Dipnoans 



Ganoids : 



Ci'ossopterygians . . . 



1 



1 



2 



5 

 1 



In studying the changes in the fish faunas it is to be noted that the 

 aberrant ostracodeims never became thoroughly established as a marine 

 order. Our knowledge of them goes back to the Middle Ordovician, but, 

 as discussed elsewhere, the scattered fragments suggest that they are 

 washings from the land into the margins of the seas. They seem, how- 

 ever, to have passed into marginal embayments; perhaps they even lived 

 to some extent in the seas; but by the Upper Devonian the waning class 

 was pretty well restricted to the land waters. Here it made its last stand, 

 but died out at the end of the Devonian. 



The elasmobranehs give their first marine record in the Silurian by 

 means of the scattered and somewhat problematic spines which are 

 grouped into the genus Onchus. This fragmentary record, as previously 

 noted, is to be compared with the relatively abundant elasmobranch re- 

 mains found in Middle and Upper Devonian marine formations. Fossils 

 of elasmobranehs, as well as higher fishes, are absent from the probably 

 brackish water deposits of the Silurian Downtonian, which hold the 

 ostracoderm remains. With the opening of the Lower Devonian the 



