Page:The Habitat of the Eurypterida.djvu/146

140 localities throughout the belt east of Langholm, showing the basal grits and greywackes projecting up through the second division of the Wenlock. This distribution indicates the presence of the basal sandstone of the advancing sea throughout the southern belt. The single eurypterid fragment found in this belt, it will be recalled, was discovered in the track-crowded shales and greywackes at Slitrig Water. These being interpreted as basal beds of an advancing sea, it is most natural to expect that the sea, rolling landwards and up the rivers, slowly but unceasingly converting the dry land into seafloor, should catch river-dwellers who were not able to or did not migrate upstream fast enough, and even if there were none such, at least dead remains would inevitably be passed over by the sea in its continued advance. One would undoubtedly expect more than a single fragment and probably more will be found in the southern Wenlock rocks. The more abundant occurrence in the Pentland Hills is explainable on the supposition that the sandy bands containing the broken exoskeletons represent the outwash from rivers into the sea, of shed exoskeletons and maybe even of the remains of eurypterids which were killed off in great numbers by the entrance of salt water into the rivers. So soon as this group of organisms was able to migrate far enough away from the sea which had overtaken the earlier individuals, the appearance of exoskeletons in that region would come to an end, but one would expect similar catastrophes to occur in another locality at a higher horizon. Unfortunately, the exact method of entombment must remain hypothetical, since the exposures are so few, but that the eurypterids did not live in the Wenlock sea is apparent. One further argument which might be adduced is that in the purely marine, open-sea Wenlock of England, not a trace of a eurypterid has been found, although if they were true marine organisms during Lower Siluric time as most geologists claim, then it is surprising that they alone of the marine fauna should be found only in southern Scotland although migration was open along most convenient marine channels into Wales.

For beauty and perfection of preservation no other known eurypterid remains can compare with those from the island of Oesel. Though only five species have been found and only one in abundance, the lack of a varied fauna is entirely compensated for by the