While some internal molds of Pelecypods are tough to figure out what genera made them, this one was easy for me to identify. It's an internal mold of a Neithea shell. It is easy to recognize because of the inequal valve sizes and the strong radial ribs that are visible. It reminds me a little of a scallop and is indeed a member of the Pectinidae.
I have specimens of Neithea from Texas that are of the external shell but I haven't yet got around to posting them. For now, enjoy this Google images search for Neithea.