Page:The Habitat of the Eurypterida.djvu/157

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Introduction. Although the Siluric of southern Scotland is characterized by large eurypterid faunas at successive horizons, the rocks of the same age in England and Wales, where more open marine conditions obtained, have yielded only two or three fragments, except in the higher Ludlow beds which mark the transition to the continental deposition characterized by the Old Red Sandstone. Even in the Ludlow the remains which in some strata are abundant show a much poorer preservation than do those from Scotland. Complete individuals are never found and although it is possible from the fragments to determine that different genera are represented, more precise identifications are difficult, and in most cases species have been erected simply in order to have some way of designating the fragments belonging to the various genera. In order to understand why all of the specimens from the Siluric of England are so much more poorly preserved than are those from Scotland, it will be necessary briefly to trace the geological changes which were taking place throughout Great Britain during the later Siluric.

In Scotland the Upper Siluric is marked by the approaching continental conditions as evidenced in the deposits of greywackes and flagstones, some barren, some containing a sparse marine fauna and others only fish and eurypterid remains. The conditions as yet were unstable, showing the alternate dominance now of river-borne sediments and now of shore deposits. To the south, however, the sea still covered most of England, though the muds pouring in from the land had made conditions unfavorable for many of the forms of life which thrived in that region during Wenlock time. Thus the corals no longer built up great reefs and only a few survived in the stifling muds wherein the graptolites were buried in such abundance. Brachiopods and the majority of molluscs likewise decreased in number as the migration to more favorable waters to the south progressed. Toward the top of the Upper Ludlow rock in England many rill and ripple-marked sandstones are to be found, some of which show trails. Near the top of this series too, occurs the "Bone-bed" which varies from $$\tfrac{1}{4}$$ inch to 6 inches in thickness and is made up largely of fish, eurypterid and crustacean remains, while a few brachiopod shells have been found in places. Geikie has estimated that this bed probably covers an