>       Since several people have picked up the Chesapecten topic, I thought I
>might add some more information.  My specimens show considerable variation.
>They begin with young shells - 2 inches and under - which are light beige,
>very scaley  (spelling, my ability to spell seems to be decreasing with my
>age?) , sand filled, and extremely fragile.  Older specimens vary from a right
>valve that is darker beige and alnost looks like alabaster, some scaling
>remaining to dark gray valves which are a lot tougher but have lost their
>scales.  All of these are definitely Chesapecten, and they are Miocene.
>I have one complete shell which is all beige, required some coating of the
>inner surface with acrylic to toughen it before I could clean the outer
>surface and measure about 3 - 4 inches.  My largest is a good 6 inchese, was
>fragile on the edges, retains some scaling, and varies from beige to darker
>gray.  These are all from the same site, age Miocene.
>        Would differences indicate some time variation, or just variations in
>the trace minerals in the ground water affecting parts of the collectig area.
>I am fairly certain that they come from a sandy cliff above the beach, as
>walking along the beach is necessary to get to the site.

As noted before by Andy Rinsberg, color variation in the shells reflects
the minerals and oxidation level they have encountered recently.  Only if a
particular deposit has had a chemical environment different from a younger
layer will color help in assigning geologic age to Chesapecten.
Development of the scales does change over geologic time, but obviously
they can get eroded as well.  Other important clues to the species and thus
the age include the number and shape of the ribs, the maximum size, and the
shape of the auricles.  Finding out what deposits are exposed in the cliff
would also help.


Dr. David Campbell

"Old Seashells"

Department of Geological Sciences
CB 3315 Mitchell Hall
University of North Carolina at Chapel Hill
Chapel Hill NC 27599-3315
USA

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919-962-0685
FAX 919-966-4519

"He had discovered an unknown bivalve, forming a new genus"-E. A. Poe, The
Gold Bug