One technical definition is to call everything Pleistocene and older (over 10,000 years) a fossil and younger but long-dead material subfossils. However, telling the difference between a well-preserved specimen just over 10,000 years and a highly altered younger specimen may be difficult. If the age is unknown, using the condition of the shell as a practical division makes sense.
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