The question is generally answered by the statement that the eurypterids originally lived in the sea and then migrated to the various marginal water bodies and estuaries where they and a few peculiar crustaceans constituted a brackish water fauna. I have already shown (p. 70) that a "brackish water" fauna consists of modified marine and freshwater euryhaline organisms with a preponderance of marine types, and that the latter show particular characteristics such as dwarfing and thinning of the shell, but that such a fauna has representatives of nearly all invertebrate phyla and is not made up of a single class of organisms. But let us assume for the sake of argument that the eurypterids and a few other arthropods did form a brackish water fauna; then another assumption is necessary, for, if a class of organisms as a whole, such as the eurypterids, should in any given geological period migrate from the sea to estuaries or other brackish water bodies and at the same time should no longer be able to live in the sea, and should not, on the other hand, become adapted to river water, then the remains of such a class of organisms should be restricted to the geological period in which the migration took place, for the class could not persist unless the estuaries persisted from period to period in the same locality (see objection to this on p. 215 below).
But since the class is known to have persisted from period to period, as indicated by the occurrences of their remains in the rocks, we are forced to conclude, on the assumption that the organisms migrated from the sea to the estuaries, that there was a persistent marine stock to repeople each successive estuary. But, if that were true, then eurypterid remains of the same or allied species should be found entombed with the marine organisms of the period in the marine equivalents of the estuarine or other brackish water deposits, and the eurypterids should have constituted a part of the typical marine fauna. But it has been shown again and again that in the contemporaneous marine deposits with typical and undoubted marine faunas, no eurypterids are found, as, for instance, in the marine Wenlock of England, or the marine limestones of the Famennian of Germany. If there is no indication of such a persistent marine stock, then there must have been a persistent stock in the rivers to repeople the estuaries in the successive geologic periods. These arguments may be applied specifically to the Siluric and Devonic of North America. During the Lower Siluric (Niagaran), the eurypterids are supposed to have lived in the sea. During the remainder of the Siluric they are assumed to
