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Three new centric diatom species assigned to a new genus are described from Miocene lacustrine deposits of Idaho. Species of the new genus, Mesodictyon, have the areola cribrum in the middle of the loculus, strutted processes and radiating, non-fasciculated striae. The strutted processes of M. magnum (diameter 60-150 μm) have long (2-3 μm) tubular extensions. The strutted processes of M. fovis (diameter 14-80 μm) are in distinct pits near the junction of the face and mantle. The valve face of M. undulatum (diameter 10-44 μm) is weakly tangentially undulate. Preliminary evidence indicates that Mesodictyon has a wide geographic distribution and may be a useful biostratigraphic marker.