Page:The Habitat of the Eurypterida.djvu/144

138 the brachiopods, pelecypods, etc.? If the eurypterids lived in the marine waters, why are their remains not found with those of the other marine organisms? Band D contains many genera, species, and individuals of brachiopods, pelecypods, gastropods, cephalopods, crustacea, crinoids, trilobites, corals, the first three groups being especially well represented, the last three by only two or three species. But not a trace of a eurypterid is found in this fauna which, according to the criteria given on p. 76 is a typical marine fauna. Moreover, why should their appearance be so sudden and so localized? The eurypterid band dies out within a few yards of Gutterford Burn, and in the other sections where one would expect to find eurypterids again, for the associated marine fossils occur, there is not even a fragment? I must confess that such anomalies in distribution are not compatible with a marine habitat for the organisms so affected. Again, why should the eurypterids have suffered such maceration, while the remains of the other organisms were entombed in a perfect state? Wonderfully preserved starfishes and trilobites are found in one of the beds, the marvellous brachiopod fauna of Band D has been described and figured by Davidson, and his book amply attests to the abundance and fine preservation of the molluscoidea; but the eurypterids are broken up, often unidentifiable and never what the palaeontologist would consider good material. These questions have been, or will be, fully treated in the appropriate sections on the bionomy and distribution of the eurypterid faunas. We are here concerned only with the lithogenesis of the beds so far as that may be separated from faunal considerations. From the study of the sections, and the lithological characters of the rocks, I offer the following interpretation for the eurypterid-bearing horizons of the Wenlock of southern Scotland.

It will be remembered that the Tarannon marked a period of retreat of the sea toward the south, the shoreline being at the end of that time somewhere in the central part of the Central Belt, while there must have been an embayment in the Girvan area where continuous marine sedimentation was active. The Wenlock sea which covered the greater part of western, central, and northern England at least, advanced over the terrestrial beds which were deposited during Tarannon time. Unfortunately, the most critical areas of deposition of the Wenlock have been removed by subsequent erosion. The Southern Belt was in the region of continuous marine deposition from Tarannon into Wenlock time, as the sections in many places