Early Miocene; Extinct.
Original Description (from Dall, 1898, p. 755-756):
"Oligocene of the Chipola beds at Alum Bluff and on the Chipola River and elsewhere in these beds; Burns and Dall.
There are several species of Amusium ranging from the Oligocene to the recent fauna in this region. In general they appear extremely similar, so much so that such figures as are ordinarily given would show no differential characters. By careful and repeated study I find myself able to separate them by the umbonal sculpture, which differs in the different forms as follows:
Nepionic shell perfectly smooth externally.
1. Shell more or less ovate: P. papyraceus Gabb.
2. Shell very large, orbicular: P. Mortoni Rav.
Nepionic left valve with obsolete radii and often feeble concentric undulations: P. precursor Dall.
Nepionic left valve with distinct flattened ribs with shallow channelled interspaces crossed by concentric, evenly spaced, not crowded, elevated lines: P. Lyoni Gabb.
Left valve of the adult with obsolete rounded ribs extending, in the adult, well over the middle of the disk: P. ocalanus Dall.
P. precursor is nearly as large as P. Mortoni, but slightly rougher and more convex when adult, the young are nearly orbicular; a distinct trace of Camptonectes striation, near the beak and submargins, may be discerned with a magnifier in a good light. Alt. 110, lat. 123, diam. 20 mm. The right valve is much flatter than the other. As the material is much broken up, it seemed hardly worth while to figure it."
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