Page:The Habitat of the Eurypterida.djvu/243

Rh p. 205 above. If we assume such a mode of distribution by rivers for the eurypterids, it would explain the close relationship which exists between forms isolated, but in neighboring localities; that is, Eurypterus lacustris of the Buffalo area, and E. remipes of the Herkimer area, nearly related species, but occurring in two isolated localities. But besides, these two occurrences, the river hypothesis must account for the close relation of both of these species to the one in the Baltic region (see below p. 235). There is good stratigraphic reason for believing that in Siluric time there was a continental mass (the Atlantica of Grabau), which as already outlined occupied much of the present North Atlantic and extended from northern North America entirely across to eastern Europe. According to Walther, several high mountain chains extended across this land connection (294, 251), and undoubtedly large rivers came down from these. Their headwaters would very probably interlace, as do those of all large rivers on the various continents at present.

Under such conditions we can see that the common ancestor of Eurypterus lacustris, E. remipes and E. fischeri could have lived in the headwaters of one of those rivers, and that getting farther away from the point of origin, the various species derived from it would be differentiated. E. lacustris and E. remipes were developed in two neighboring streams, but the forms connecting E. remipes and E. fischeri which must have lived in the rivers of Atlantica, are now buried under the waters of the North Atlantic Ocean. The more distant relationship of these species suggests that there were intermediate forms, though these have not yet been found, and are probably nowhere preserved, though it is not impossible that Siluric strata with such intermediate species may exist beneath the ice cap of Southern Greenland. In this great system of rivers, which to all appearances characterized the continent of Atlantica, the Bertie and Herkimer Rivers were not very far apart, so that the faunules of each were very similar. In fact, the deltas spread out at the mouths of the two rivers may have become confluent in their outermost or seaward portions, though the waterlime now known would, as above explained, represent only the inshore facies. It may have happened that in times of flood the river waters flowed out over a broader area near the debouchures until some of the distributaries became for a time confluent, thus allowing some of the species from one river to be carried over into the area of deposition of the other. Thus might the presence of Pterygotus cobbi in both regions be accounted for.