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
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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