Yesterday I hunted an Upper Santonian Austin Chalk site in Ellis county between downpours of rain. This was my first time at the site. I was looking around a pile of rocks with some boulders mixed in and found this on one of the boulders. It is tiny, whatever it is, about 1 cm wide. Now I must admit that I am more familiar with the fauna of the Upper Coniacian Austin Chalk but to me this looks more like half of the aptychus of an ammonite than a bivalve. Since an aptychus was made of calcite I believe that it would be preserved in chalk, though the actual fossilized material is gone and this is just the steinkern of what ever it was.
Here is the best picture that I have of it. Sorry that the quality is poor. I took it while I was at the site and I can’t get a better picture right now. Hopfully this will be sufficient.