True Eucrassatella goes back to the Paleocene, but the often similar Crassatella extends into the Cretaceous.  From your description, I assume that you found an actual shell embedded in sandstone rather than a mold.  In this case, it is probably mid to late Pliocene or Pleistocene.  There are several species, though usually only one or two at a time.  The older deposits in that general area (extending back to the Cretaceous) usually have the aragonitic shells dissolved away, leaving only molds.  Some of these layers do have calcite casts, however.  They are usually slightly to very brown and coarsely crystalline.  Ward has split the Miocene-Pleistocene lineage in the southeastern U.S. from Eucrassatella, calling it Marvacrassatella; this genus is not recognized by L. Campbell, 1993.  The subgenus Hybolophus is also used for the local species; its type is Panamic.

Plio-Pleistocene species and subspecies recorded from the U.S. Atlantic and Gulf Coastal Plain are

Eucrassatella alaquaensis (Mansfield)
Eucrassatella kauffmani (Ward and Blackwelder)
Eucrassatella meridionalis (Dall)
Eucrassatella speciosa (A. Adams)
Eucrassatella virginica (Gmelin)
Eucrassatella virginica cycloptera (Dall)


Incidentally, a Pliocene Eucrassatella (E. virginica) from Virginia seems to be the first figured (Lister, before the Chesapecten jeffersonius and Echora quadricostata) and first described (Gmelin) fossil mollusk for the U.S.

    Dr. David Campbell
    "Old Seashells"
    Biology Department
    Saint Mary's College of Maryland
    18952 E. Fisher Road
    St. Mary's City, MD  20686-3001 USA
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"Mollusks murmured 'Morning!'.  And salmon chanted 'Evening!'."-Frank Muir, Oh My Word!