An unusual trochiform gastropod from the upper Ordovician of Sweden

unusual trochiform gastropod, Semizona bella gen. et sp. nov., is described from the Boda Limestone carbonate mounds (upper Ordovician, Ashgill) of ce ntral Sweden. A second species, S. glindmeyeri (Rohr, 1996), is recognised from the Ordovician (Whiterockian) of Nevada. The shell shape and the stron gly prosocline tangential aperture of Semizona suggest that balancing of th e shell on the head-foot mass was accomplished by tilting of the axis of co iling of the shell to about 65 degrees with 10-30 degrees of regulatory det orsion. The rounded aperture allowed straight contraction of the retractor muscles, suggesting clamping behaviour often associated with a sedate, graz ing snail. This agrees with the environmental setting, which suggests a har d substrate with rich microbial growth. Besides clamping, the subsutural no des and thick shell were probably effective against predation; repaired inj uries indicate failed predatory attacks. Semizona shows morphological simil arities with some pleurotomariin vetigastropods, and with the family Pseudo phoridae Miller, 1889.