THE SILURIAN PERIOD. 



413 



are marine. In the transition beds of England, Sweden, and Russia, 

 the eurypterids are associated more freely with marine forms, but they 

 are also associated with the seeds of land plants and with fish which, 

 in the succeeding stage, seem to have occupied land waters chiefly. 

 In the Devonian and Carboniferous periods, in which the eurypterids 

 reached their climax and passed into their decline, and where they 

 seem to have been in their more normal relations, they are associated 

 with land plants, scorpions, insects, fishes, and fresh-water amphibians, 



Fig. 189. — a, Eurypterus fischeri Eich., restoration after Schmidt ; b, Pterogotus anglicus 

 Agass., restoration after Woodward; c, PaUeophonus caledonicus Hunter, after 

 Peach; d, Palceaspis americana Claypole, restoration by Claypole, modified by 

 Dean. 



which seem to imply a fresh- water habitat. In the light of these 

 facts the more common inference has been that they were originally 

 marine forms, and became adapted later to brackish and fresh-water 

 conditions. 1 The alternative inference is that they were originally 

 denizens of the land waters, and that their remains were occasionally 

 and sometimes quite freely carried out to sea by streams, and were 



1 See Zittel's Text -book of Paleontology, Eastman's Translation, 1900, p. 673. 



