Page:The Habitat of the Eurypterida.djvu/239

Rh Baltic area that the latter for a long time was identified with the former. Schmidt was the first to suggest that the differences between the two species were only geographical variations arising through migration. With this idea Clarke and Ruedemann have concurred. In fact, they point out in many places the close similarity between the Bertie and Oesel fauna, and especially between the two commonest species in both, E. remipes and E. fischeri. Now, if the Bertie fauna was an estuarine one, preserved from Pittsford time in various brackish water bodies, when and how did migrations take place to the Baltic sea of the Upper Siluric? The answer will undoubtedly be that the members of the marine stock in the Lower Siluric which were not caught or did not voluntarily seek refuge in the "lagoon" or remnant of Niagaran sea in New York State, migrated along the shore of Atlantica, passing from estuary to estuary until they reached, the island of Oesel. This might seem like a very happy solution, if the British Isles did not intervene between America and Oesel, and if they did not have a very clear record to show that no such migration took place. In the discussion of the faunas of the various Palæozoic continents, given below, it will be shown that the Wenlock, Ludlow, and Lanarkian faunas of Great Britain offer no indications of migrations along the neritic zone during those periods and that in many cases new genera as well as new species arose suddenly without, apparently, having a genetic relationship to corresponding taxonomic units in other countries.

Since the assumption that the early Salina eurypterids lived in a "lagoon more or less cut off from the sea," leads to such difficulties, we must seek another theory. Let us assume that they lived in the rivers, and draw the logical deductions. It has been shown from their lithogenesis that the Pittsford and Shawangunk deposits must have been derived from Appalachia, while the Bertie was derived from Atlantica. Rivers, whether existing at the same time or at different geological periods, would carry related forms if coming from the same continent, but unrelated or only distantly related forms if coming from two different continents. Thus the Pittsford and Shawangunk eurypterids would be near relatives to say the least; while the fact that the larval forms from the latter are merely the young of those from the former is all the more according to our expectations. Likewise the absence of close relationship between the Shawangunk and the Bertie,