The use of growth lines for dating can be (and has been) tested using stable isotopic variations.  In most environments, there is a significant seasonal variation in the ratios of oxygen 16 to oxygen 18 and carbon 12 to carbon 13.  The growth bands generally correspond well with the isotopes, though the environment must be considered.  E.g., Mercenaria quits groing in summer in Florida and in winter in New England.   Margaritifera margaritifera in the northern part of its range can also exceed a century, and deepwater species also appear to grow extremely slowly and live very long.

The geoduck is one of the few mollusks to be a school mascot.  Evergreen State, in Washington (not DC), has plastic geoducks and silly geoduck T-shirts available in the school store.

    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