Early Miocene; Extinct.
Original Description (from Dall, 1898, p. 725):
"Upper Oligocene (Alum Bluff beds) of Oak Grove, Santa Rosa County, Florida; on the Chattahoochee River at Old Chattahoochee Landing, at Rock Bluff, and on the Chipola River in the Chipola beds.
This form is the precursor in the Upper Oligocene of the typical Madisonius of the Miocene. It differs from the latter in its extreme compression, the ribs, except in the umbonal region, being almost obsolete. Alt. 120, lat. 135, diam. about 16 mm. This is what has been referred to Jeffersonius and Madisonius by L. C. Johnson, Foerste, and other observers in the Floridian Oligocene.
The characteristics of P. Madisonius [var. Sayanus] have been pointed out under P. Jeffersonius, from which it differs by its more compressed shell, more numerous ribs, and coarser and more scabrous sculpture, as well as the deeper and wider byssal notch. The young rarely have the scales continuous across the tops of the ribs."
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