656 



GEOLOGY 



unknown forms. Aside from the developments of the fresh-water 

 fish and of the amphibians, perhaps the most suggestive feature 

 was the association of the arthropods with other forms of life. 



Eurypterids (Fig. 455) were still in 

 existence, and their relics are so 

 intimately associated with beauti- 

 fully preserved ferns, calamites, in- 

 sects, spiders, and scorpions as to 

 leave no reasonable doubt that they 

 were fresh-water forms. There were 

 also crustaceans resembling crayfish, 

 and others of shrimp-like appear- 

 ance. 



IV. THE MARINE LIFE 



Because of the approximation of 

 great areas of the continent to sea- 

 level, two phases of sea life had 

 sufficient prevalence to be worthy 

 of note. The first consisted of those 

 forms that habitually occupied the 

 thin edges of the sea, which, in the 

 form of estuaries, lagoons, and shoals, crept in and out on the 

 borders of the continent as the relations of land and sea oscillated, 

 while the second embraced the life of the more open seas. This 

 distinction had doubtless always existed, but it had not before 

 reached equal importance. Life of the first phase was not usually 

 well preserved, but in the coal regions of this period, it consti- 

 tutes the better part of the record. In this phase, where sandy 

 and muddy flats prevailed, pelecypods and gastropods, together 

 with certain fishes, predominated, while in the more open seas the 

 brachiopods, cephalopods, and the clear-water types were more 

 plentiful. 



It is difficult to tell which of the fishes should be regarded as 

 marine, which as fresh water, and which as common to salt and 



Fig. 455. Natural association 

 of Eurypterus mansfieldi with 

 ferns and calamites. (From 

 Dana, after Hall.) 



