Art,
  I would propose the genus Chesapecten WARD & BLACKWELDER 1975 as a set
of taxa which show evolutionary trends.  I believe the earliest on is C.
coccymelus from the early Calvert Formation (miocene) which then goes thru
various species until it dies out in the Pliocene with the last two taxa
being C. madisonius and C. septenarius.  If you need more details, ie. the
full list of taxa and the geological horizons that they come from I can
pull it out of my library.  Let me know.
 
Charlie
************************************************************************
 
Charlie Sturm, Jr
Clinical Instructor - Family Medicine
  University of Pittsburgh School of Medicine
Research Associate - Section of Invertebrate Zoology
  Carnegie Museum of Natural History
 
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On Mon, 2 Mar 1998, MR ART WEIL wrote:
 
>    Two questions. 1. I have a small pecten, which at first glance I
> mistook for an operculum. Same color, same thinness. It measures 16.
> 4mm high and 18.9mm wide. Very delicate. There are barely visable
> lighter spots on the shell.Could it be a juvenile? It comes from the
> Thomas Say collection via Columbia College.
>   2. Evolution: Are there any existing shells that we can deffinately
> say developed from an exact species in Earlier Pleistocene, and then
> back to some earlier species back in (say) Miocene? When we find
> fossils, we know that there are similar existing species, but are
> they traceable (DNA?) to some exact species in the past or just to a
> general family?
>   3. See!? No pigs.
>                    Art
>