require an identical answer, but the singular association of the two in unusual display under peculiar conditions and on both continents strongly implies a community of habit, at least at the stages in question. The association is one of the most unique [!] faunal and physical combinations of geologic history.
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 Precambrian 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 infrequently found in marine beds; but transportation in the opposite direction can not be assigned . . . From the occurrence of eurypterids first in marine beds apparently and later in fresh-water deposits it has been inferred that they were originally sea dwellers and later became adapted to land waters, 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.
In view of the contrasting opinions which have been thus expressed as to the original habitat of the eurypterids, it will be well to analyze closely here the evidence from the rocks which contain the eurypterids, and from the associated species.
In regard to the Cambric Strabops thacheri, the Lower Siluric Echinognathus clevelandi and Megalograptus welchi, the Clinton Eurypterus prominens and the Guelph E. boylei, we might concede, in view of the fact that all these remains have been found in only a single individual each, that they are remains carried out to sea from terrestrial waters, yet their combined evidence inclines to the side of the marine habitat.
The profuse Lower Siluric (Frankfort) fauna is associated with seaweeds, graptolites, trilobites, cephalopods and brachiopods, and inhabited the pools of a littoral region with abundant detrital sediments. All
