Early Pliocene to Middle Pleistocene; Extinct.
Southern Florida to Virginia.
Original Description (from Tuomey and Holmes, 1857, p. 29):
"Description. Shell orbicular, convex, somewhat thick, equivalve, with concentric lines of growth, ears nearly equal; lower valve, buccal ear notched, radiately and coarsely ribbed, with five to six ribs; anal ear ribs smaller and more numerous; upper valve, ears with the radiating lines equal; ribs and interstices nearly equal.
This fossil is distinguished from P. eboreus by having the ribs more raised and convex, both valves more convex, and by the coarse ribs of the buccal ear of the lower valve. Like P. eboreus, it is also a variable shell, some specimens nearly equilateral, and others very inequilateral. It also bears a strong resemblance to a recent shell from the coast of Florida.
Plate XL, Fig. 6. Upper valve, natural size.
" 7. Lower valve.
" 8. Side view of shell.
" 9. Outline of ribs.
" 10. Magnified view of a section of the ribs and interstices, showing the concentric undulating lines.
Locality. Darlington; Smith's, Goose Creek."
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