The lymnocardiines and dreissenids are, among the bivalves, the most prominent of the Paratethyan groups.  Northward movement of Africa, Arabia, and India closed off the Tethys seaway, a tropical ocean extending across the southern edge of Eurasia, in the Miocene.  This created giant brackish lakes in Europe and Asia; the Black, Caspian, and Aral seas being present-day remnants.  Along with gastropods and other organisms that managed to adjust to the changed salinity, these groups were able to diversify into a wide range of forms.  For example, the only known freshwater cave bivalve is a dreissenid in the Balkans.

    Dr. David Campbell
    Old Seashells
    University of Alabama
    Biodiversity & Systematics
    Dept. Biological Sciences
    Box 870345
    Tuscaloosa, AL  35487-0345 USA
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That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droitgate Spa