Silurian oncocerid cephalopods from Gotland

Two new genera, Jeppssonoceras and Plemeroceras, eight new species, Trimeroceras bulbosum, Pentameroceras facula, Clathroceras plieatum, Jeppssonoceras concentratum, Plemeroceras cassis, Oetameroceras sinuosum, O. pugil and O. unguifer, and four old species of oncocerids are described from Gotland. Comparative studies are made on material from Czechoslovakia, Great Britain, Estonia and the United States. Immature and mature growth stages are discussed. Growth patterns in the apertural area support the interpretation that the number of sinuses does not necessarily reflect any specific genus, or species. Sexual dimorphism based on size and shape of shell is demonstrated in six species. Truncation seems to occur in three species. Floating conditions and swimming abilities are discussed as well as colour patterns. Comparisons are made with the extant Nautilus and Spirula. Malformation and boreholes in shell are described.