Hello, Conch-Lers!

I have been working on identifying the bivalves in the collection
the club bought.   I have run accross a shell which is identical
to the picture in Eisenberg (I know this is not one of the best
books) of "Protothaca grata" Say.   There is also another shell
that I am sure is the same - outer clor and pattern different,
but everything else the same.

The trouble is that I can't find this namee in any of my other books.   Is it valid, or is it a synonym, and if so, of what?

Thanks,
Linda