Page:The Habitat of the Eurypterida.djvu/110

 Sarle to be a plant. Upon this bed lies the shaly marlite, the first of the lowest Salina or last of Upper Niagaran, and above this a waterlime.

There is thus a marked change in the physical conditions which accompanied the withdrawal of the Niagaran sea and the initiation of the Salina type of deposits. In the lowest bed of the series occurs the last of the Niagaran fauna, ''Pterinea cf. emacerata''. In the waterlime (no. 14) the same species occurs, together with a Lingula, ''Leperditia cf. scalaris'' and an Orthoceras. Then not again till the black shale of the eurypterid horizon, do these forms appear, but it is of great significance that these pure marine fossils do not occur in the eurypterid shale beds proper, but in the thin dolomite partings, and that in these partings the eurypterids are almost entirely wanting. The dolomites were evidently marine, but the conditions under which they were deposited "were not favorable to the eurypterids" says Sarle (240, 1086). The question immediately arises: why were the conditions not favorable? and then, where were the eurpterids during the intervals between shale deposition, i.e., during the time of marine dolomite deposition? If the eurypterids were inhabitants of the marine waters or of bays, estuaries, etc., they should be found in the beds containing the littoral fauna of the impure dolomite, for, as has been shown in Chapter III, the faunas of bays and other indentations along the shore are not restricted to such areas, but are much the same as the faunas extending all along a continental shore fine. Since the eurypterids were not living in the marine waters, where were they when there were no black muds forming? They appear suddenly in countless numbers representing six species, and they come with the black shale facies and disappear again just as suddenly. Their range, too, is very small. Sarle says: "Though the fine character of the silt forming the black shale and the evidence of interrupted sedimentation noted above, indicate slow accumulation, the occupation by the eurypterids was apparently of comparatively short duration, merely an incursion, as it were, since the black shale all told does not exceed 2 feet in thickness" (240, 1086).

To determine where the eurypterids were before their two sudden appearances, one must turn to the source of the black muds. These were not deposited in the open sea, for marine forms are wanting, and in any case, the mud must have been derived from the land. At that time the land to the west, north and south was all covered