THE DEVONIAN PERIOD. 479 



some reserve. They seem to have been of the same general types 

 as in the preceding epoch, but with a decline in the singular arthro- 

 dirans, an increase in the sharks, and a continuation of the ganoids 

 without assignable change. 



The pelagic and abysmal life of the Devonian. — Concerning the 

 pelagic life of the Devonian nothing is known except as it may be 

 obscurely suggested by the floating forms that were entrapped in 

 the sediments of the shallow-water zone, or that found their way into 

 the interior seas, and this element does not admit of satisfactory inter- 

 pretation. 



Respecting the abysmal life nothing is known. The strata reveal 

 no clear case of beds laid down on the abysmal bottom of the Devonian 

 ocean and subsequently lifted to an accessible position. It is therefore 

 important to keep ever in mind the limitations to which all statements 

 respecting the marine life of this period, if not of all geological periods, 

 are subject. 



Marine vegetation. — As usual, the record of the marine plants is 

 too imperfect for satisfactory discussion. There are evidences of 

 algae as before, and they appear to have been well deployed, and theo- 

 retically they must have been abundant. Sections of what are inter- 

 preted as the stems of a gigantic seaweed (Nematophycus), sometimes 

 reaching three feet in diameter, have been found. They perhaps mark 

 the dimensional climax of this form. 



The Devonian Fauna in the Great Basin Area. 



In the region which constitutes the Great Basin of the west, a 

 large area seems to have been occupied continuously by the sea from 

 about the beginning of the Devonian time to the later part of the 

 Carboniferous period. It seems to have been measurably free from 

 both the physical and the biological changes which gave such diver- 

 sity to the eastern provinces. The fauna seems to have had a slow 

 continuous evolution, fed, it would appear, from time to time by acces- 

 sions from the two northern provinces or from some source with which 

 they were connected. None of the distinctive South American forms 

 appeared in it, nor any of the peculiar Helderberg or Oriskany forms. 

 It is inferred, therefore, that it was disconnected from these provinces 

 throughout the whole Devonian period. On the other hand, a not a- 



