ID for odd fossil from quarry in Austin Chalk area South of Dallas Texas?

[rickeclectic]

Hi,  Any help greatly appreciated. Went on a club trip with the Dallas club, mostly looking for shark teeth in a club sponsored trip to a private cement quarry south of Dallas, Texas. Found shark teeth (Scapanorhynchus, Squalicoraxs, Ptychodus), but also found this odd  thing. I have cleaned all the extraneous material off of it (with a dremel using a wire brush) and the remaining is hard and fossil like, but a real puzzler. I have included a back and front photo. It was in gray material that had phosphate nodules and small shark teeth.

 

Thanks,

 

Rick

[LSCHNELLE]

I am no expert on Cretaceous oysters, but this is clearly an oyster. In the Austin chalk, it might be a Pycnodonte species. Or, maybe something of similar Late Cretaceous Age. The first picture of the anterior side gives away that it is a bivalve. The posterior view ridges suggest an oyster.