34 THE HABITAT OF THE ETJRYPTERIDA 



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 Palceophonus nuncius, 

 a scorpion. Lindstrom has called this thin stratum the Pterygotus 

 marl, and it is seen to lie just at the top of III. 7 Here there is a break 

 and disconformity, above which follows a conglomerate (IV) with 

 waterworn gastropoda and portions of Spongio stroma holmi Roth- 

 pletz. 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 Stromatoporae 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 Karlso 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 dolo- 

 mite, 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 Rootzikiill, 

 on the west coast of the island, in the parish of Kielkond. Here 

 the beds are a dolomite in which the chitinous exoskeletons of Ptery- 

 gotus 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 de- 

 stroyed, 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 remain- 



7 Professor Grabau has argued that this bed should be placed in the Upper Gotlandian. 



