BUFFALO SOCIETY OF NATURAL SCIENCES 143 



schichten von Rootzikiill 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. 



General Stratigraphy. 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 direc- 

 tion. 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 swindernana, a coral found 

 in the Upper Visby beds of Gotland, also Leperditia baltica, which 

 occurs in divisions V, VI and VII of North Gotland, Strophoinena 

 imbrex, found throughout the Wenlock or lower divisions in Gotland, 

 and Zaphrentis conulas, 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 



