>  Now, regarding Chesapecten jeffersonius from North Carolina: do the two valves have different numbers of ribs?

I don't seem to have any pairs on hand, but from the handy figures, the ribs appear to be actual plications-one rib corresponds with the interrib of the other valve.  Thus, the number should match (though counting the small ribs towards either end gets a bit tricky).

There are C. jeffersonius-C. madisonius transitional forms in rare mid-Pliocene outcrops (Lower Goose Creek Limestone equivalent; only a handful of known outcrops, several no longer extant) that have an intermediate numer of ribs in both valves.  C. septenarius in South Carolina has a variable rib number in the Goose Creek Limestone, in contrast to its normal constant 7 rib form.

    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

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