Page:The Habitat of the Eurypterida.djvu/38

 ripple-mark structures are sometimes observable. Above these strata comes a complex of layers, one meter thick, consisting of marl shales and limestones with Pterygotus osiliensis and Palæophonus nuncius, a scorpion. Lindström has called this thin stratum the Pterygotus marl, and it is seen to lie just at the top of III. Here there is a break and disconformity, above which follows a conglomerate (IV) with waterworn gastropoda and portions of Spongiostroma holmi Rothpletz. The relations of the reef limestone and marl are well shown in the vicinity of Visby. The reefs are composed of non-stratified accumulations of Stromatoporæ mainly, with a few corals in addition. Between the reefs of II are the finely stratified bituminous, brownish shales of III, well shown on the island of Karlsö west of Visby, which contain a marine and estuarine fauna mixed.

Upper Siluric of Oesel. This island has yielded a large crustacean fauna in the usual association with eurypterids. Two species of Eurypterus are reported: E. laticeps Schmidt from two fairly perfect head shields, and E. fischeri Eichwald from many excellent specimens. There is also an abundance of fragments of Pterygotus osiliensis. The bed in which these occur is a fine grained Platten-kalk or dolomite, with a peculiar fauna throughout; this is followed by other granular limestones containing the usual uppermost Siluric fauna. The Eurypterus beds have a fairly wide extent in western Oesel, but the fullest development of the fauna is seen only near Rootziküll, on the west coast of the island, in the parish of Kielkond. Here the beds are a dolomite in which the chitinous exoskeletons of Pterygotus and Eurypterus have been excellently preserved, and even the tail sting of a Ceratiocaris and the shields of two cephalaspid fishes, Thyestes verrucosus Eichw. and Tremataspis schrenckii Schmidt, and the shells of the little Lingula nana Eichw. have been found. Rather rarely occurring are the Hemiaspidae: Bunodes lunula Eichw., B. rugosus Nieszk. and schrenckii Nieszk. sp. as well as Pseudoniscus aculeatus Nieszk. and the shells of Orthoceras tenue Eichw. Bunodes and Leperditia are represented by many specimens, but these and all the other fossils mentioned show, in place of the shell which is destroyed, only a black carbonaceous film representing the organic material (Schmidt 248, 28). The eurypterids do not show the same kind of preservation, for Schrenck (254, 35) reports the integuments of Eurypterus remipes Dekay (with which E. tetragonophthalmus Fischer is synonymous and which Schmidt has since placed under E. fischeri,) to be entirely unaltered, not only chemically, still