Where was it found?   There are silicified deposits
throughout the geologic record, so it could be just about
anywhere from the Ordovician to the Pleistocene.  Although
the Cardioidea first appear in the Triassic, there are
unrelated Paleozoic species with enough superficial
resemblance to cardiids to have been called Cardium back
in the 1800's.

    Dr. David Campbell
    Old Seashells
    University of Alabama
    Biodiversity & Systematics
    Dept. Biological Sciences
    Box 870345
    Tuscaloosa, AL  35487-0345 USA
    [log in to unmask]

That is Uncle Joe, taken in the masonic regalia of a Grand
Exalted Periwinkle of the Mystic Order of Whelks-P.G.
Wodehouse, Romance at Droitgate Spa