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Abstract
A new genus, Trossulus Ross & Sims, is established for four fossil species of the Heliopeltaceae which are very similar to both Actinoptychus Ehrenberg and Polymyxus L.W. Bailey in the contour of the valve but which have rimoportulae very different from those of all other genera of the family. These rimoportulae open externally by one or more radial slits above a shallow chamber and internally by a lipped, straight or slightly sinuous, more or less radial slit. Actinoptychus elegantulus Grove & Sturt is transferred to Trossulus as its type; a neotype for this species is designated. Actinodiscus californicus Hanna is also transferred to Trossulus. Two new species, T. nicobaricus and T. javanicus, are described; both are based on specimens previously misidentified as Actinoptychus elegantulus. Trossulus ranges in age from the middle Eocene to at least the middle Miocene.