To prepare a figure for a manuscript I am writing, I photographed some shells today. The shells were of 2 species found at one locality that we surveyed in Turkey last year. I wanted to show how superficially similar the shells of the 2 species are.
The shell marked A is Discus rotundatus, while the shells B and C are juvenile and adult, respectively, Pleurodiscus balmei. Actually, the fresh shells of the former species have reddish radial bands that make it easy to identify it. The shell pictured here, the best one I could find at this particular locality, is old and faded (more so is the juvenile P. balmei). The adults of D. rotundatus are also smaller than those of P. balmei.
Discus rotundatus is a European species that is occasionally encountered in Turkey, while P. balmei is native and quite common in western Turkey.