The fauna of the Baltoscandic Orthocerataceae is important because it fills a documentary gap between the Lower Ordovician fauna of North America and the upper Middle Ordovician fauna of China and North America. A revision of the Orthoceratidae, Geisonoceratidae, and Arionoceratidae is given on a material of more than 450 specimens. Intraspecific and ontogenetic variations were observed. The following new taxa are erected: Plagiostomoceras fragile sp. nov., Archigeisonoceras repplingense sp. nov., Archigeisonoceras picus sp. nov. Archigeisonoceras folkeslundense sp. nov., Nilssonoceras latisiphonatum gen. et sp. nov., Kinnekulloceras kinnekulense gen. et sp. nov., and Arionoceras lotskirkense sp. nov. The genus Archigeisonoceras is described for the first time in the Baltoscandic area and the oldest occurrence of the genus Arionoceras is documented from the Middle Ordovician of Baltoscandia. It is shown that the endosiphuncular deposits of the Middle Ordovician orthoceratacaceans are highly variable and widespread in the apical parts of the phragmocone.

Key words: Cephalopoda, Nautiloidea, Orthoceratacea, Ordovician, Baltoscandia.

Björn Kröger [buxcreau@gmx.de],Geologisch−Paläontologisches Institut und Museum, Universität Hamburg, Bunde−sstr. 55, D−20146 Hamburg, Germany.