Page:The Habitat of the Eurypterida.djvu/149

Rh von Rootziküll auf Oesel." Precise information is given regarding localities, species are fully described and compared with related forms and excellent illustrations are given, so that with these papers and one by Nieszkowski in 1859 on "Der Eurypterus remipes aus den obersilurischen Schichten der Insel Oesel" (197) one may gain an accurate knowledge of the fauna and the sediments. Notes by Nieszkowski in connection with his work on the trilobites have proved helpful, and for further details the reader is referred to the titles under his name in the bibliography as well as to numerous papers by Schmidt.

. The Siluric exposures on the island of Oesel include two divisions: the lower Oesel group or zone I, and the upper Oesel group or zone K of Schmidt. The strata have a gentle dip to the south so that higher and higher beds appear in that direction. The lower beds, of Wenlock age, cover a considerable part of the northern half of the island, while the upper or Ludlow beds are found in the southern portion (see map, fig. 13). In the extreme north the lowest part of zone I occurs carrying typical Wenlock fossils; southward, as on the peninsula of Taggamois the upper division of the zone is exposed, showing well its dolomitic reef structure; bryozoa, crinoids and brachiopods are abundant, and do not differ essentially from the forms in the underlying marls. The last exposures of the upper part of zone I yield abundant Thecia swindemana, a coral found in the Upper Visby beds of Gotland, also Leperditia baltica, which occurs in divisions V, VI and VII of North Gotland, Strophomena imbrex, found throughout the Wenlock or lower divisions in Gotland, and Zaphrentis conulus, characteristic of the upper part of the Visby formation (III) immediately below the Pterygotus marl of Gotland. This higher portion of zone I is to be correlated with the Leperditia baltica zone of Gotland (Schmidt, 250, 132).

Throughout the entire south and southwestern parts of the island, zone I is succeeded by zone K, but the actual contact is nowhere observable. This zone likewise shows two subdivisions, a lower, made up of thin-bedded "plattenkalk" or dolomite, in some places unfossiliferous, in others carrying eurypterids and fishes, and an upper very fossiliferous horizon known as the Ilionia beds on account of the abundance of that pelecypod. The Ilionia, beds are to be correlated with zone VI of Gotland which is of Upper Ludlow age. Some of the diagnostic Upper Ludlow fossils recorded from this horizon in Oesel are: Ilionia prisca, Megalomus gotlandicus, which occurs just above