Page:The Habitat of the Eurypterida.djvu/264

 On the other hand, these peculiarities are easily understood, if we consider the eurypterids as fluviatile organisms. It is quite evident that the Kokomo deposits have a distinctly different source from those of the Bertie. If then, these eurypterids belong to a distinct river system, developed upon a separate land mass, it would indeed be surprising if they were not wholly distinct specifically from those of the rivers of Atlantica which were responsible for the Bertie waterlime deposits. The alternation of beds with marine fossils with beds carrying only eurypterids and ceratiocarids, suggests that the Kokomo deposits may have approached those of some modern estuaries in which we have an alternation of marine and fresh-water deposits. The map, Fig. 8, shows the position and general extent of these late Siluric river systems.

When the significance of the distribution and of the occurrence of the eurypterids is given its full importance, there can no longer be any doubt that the eurypterids at no time of their known history were normally marine organisms. We cannot conceive of marine animals presenting such localized occurrences and yet having such wide distribution as a class. The question of transit seems not to have been considered by previous authors, and yet it is one of the greatest importance. If we suppose that the eurypterids lived in the Palæozoic rivers, we have furnished them with the proper milieu for individual as well as racial development. For we must not overlook the fact that when these animals make their appearance in numbers, they are already highly differentiated. To a river dweller migrations from the headwaters of one river system to those of another are easily possible, and this is the only way by which we can account for the distribution of these organisms, unless we assume migrations along continuous shore lines which is, however, negatived by the lack of remains in the shore deposits of the period in which they most abound. Furthermore, the segregation into "pools" can be accounted for only by assuming that these "pools" were fed each by its own river system. A candid and unbiased survey of the facts presented cannot but lead to the unqualified belief in the fluviatile habits of these remarkable arthropods of the Palæozoic era of the earth's history.