Mesodictyon, a New Fossil Genus of the Centric Diatom Family Thalassiosiraceae from the Miocene Chalk Hills Formation, Western Snake River Plain, Idaho

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Abstract
Three new centric diatom species assigned to a new genus are described from Miocene lacustrine deposits of Idaho [USA]. 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 .mu.m) have long (2-3 .mu.m) tubular extensions. The strutted processes of M. fovis (diameter 14-80 .mu.m) are in distinct pits near the junction of the face and mantle. The valve face of M. undulatum (diameter 10-44 .mu.m) is weakly tangentially undulate. Preliminary evidence indicates that Mesodictyon has a wide geographic distribution and may be a useful biostratigraphic marker.