HABITAT OF THE EARLi VERTEBRATES 403 



The earlier occurrence of the Eurypterids in marine deposits 

 is almost as limited as that of the fishes, and yet they were well 

 adapted to fossilization and were actually fossilized as far back 

 as pre-Cambrian times, as Walcott has recently shown by their 

 discovery in the Belt Mountain terrane of Montana. Of about 

 a dozen known genera of Eurypterids, only two or three of 

 those least well known are without associations with formations 

 regarded as fresh water. The relics found in marine sediments 

 may be attributed to transportation from the land just as is done 

 in the case of the terrestrial plants and land insects not infre- 

 quently found in marine beds ; but transportation in the opposite 

 direction cannot be assigned. In the Ordovician but a single 

 Eurypterid representative is known to occur and of this very 

 little is known. In the pre-Cambrian beds of Montana a more 

 abundant presence seems to be indicated, but little has yet been 

 learned of the concurrent physical conditions. The thousands 

 of Crustacean fragments are associated with a few trails assigned 

 to annelids and some that are possibly molluscan or crustacean, 

 and the inference is that the deposits were made in the sea. 

 From the occurrence of Eurypterids first in marine beds appar- 

 ently and later in fresh-water deposits it has been inferred that 

 they were originally sea-dwellers and later became adapted to 

 landwaters, but the meagerness of their marine record on the 

 one hand, and their abundance and fine preservation in the fresh- 

 water deposits on the other, give point to the question whether 

 their early marine record is anything more than the chance 

 deposit of river forms borne out to sea. When it is considered 

 that the records of acknowledged marine types are, on the whole, 

 good as such things go, and have been widely and well-studied, 

 there is an incongruity in the case of both the fishes and the 

 Eurypterids between the meager marine records of the Ordovi- 

 cian and Silurian, and the impressive fresh-water record of the 

 same forms in the Old Red Sandstone phase of the Devonian, 

 and this incongruity may well be regarded as significant. 



There is reason to believe that opinion has been much influ- 

 enced — more or less unconsciously no doubt — by general 



