brachiopods (Orthis elegantula, etc.) and the so called Ostergarn beds which still contain Eurypterus, but also Lucina (Ilionia) prisca, Meristina didyma, Leperditia and in their uppermost layers species of Chonetes, Spirifer, Beyrichia, etc.
Thus the Eurypterus beds of the Salina formation in which the fauna reaches its climacteric development, are clearly stamped with their marine origin, and the profusion and perfection of preservation of the eurypterid remains precludes the possibility of their transportation into the basin by land waters; it is also apparent that the beds were not formed under normal marine conditions. The eurypterid horizons of Kokomo, of the Salina and of Oesel, as well as of Great Britain, exhibit as clearly all the characteristics of a particular and peculiar marine facies, as do the graptolites or the corals. These facies are indicated partly by the unusual nature of the rocks and partly by the peculiar aspect of the associated
lowed after an interval by several hundred feet of shales and sandstones that form the top of the Frankfort formation and contain a distinct fauna without eurypterids. These rocks, elsewhere designated the Indian Ladder beds, are typically exposed in a high bluff below the Indian Ladder at the Helderberg escarpment.
The peculiar restriction of the eurypterids to the easternmost exposures of the Frankfort shale would seem at first to be explicable, as in the case of the Salina faunas, by assuming their occurrence in "pools." But the Frankfort shale exhibits notable differences from the Salina in total thickness, faunal association and lithological character as between the eastern and western occurrences, evidence which tends to indicate that the eastern beds were deposited close to the shore line, and the more western beds farther offshore. The Frankfort eurypterids were thus inhabitants of the shallow littoral waters with their mud flats and lagoons, and if the shore line of the late Utica-Frankfort period extended in northeast-southwest direction, just east of the lower Mohawk region, as conceived by Schuchert [Paleogeography of North America, pl. 60] the exposures along the lower Mohawk valley happen to intersect only their former narrow habitat. The range of the eurypterids extends in this limited area through a thickness of from 1500 to 2000 feet which is due to the fact that this region was apparently involved in the Appalachian folding then going on, and thereby suffered depression while large quantities of material were swept down from the rising land in the east.
