Possible exogyra fossil?

[ieatshinyrocks]

Found several years ago in an area of Cibolo Creek that cuts through the Pecan Gap Chalk near San Antonio. 
 

After some brief research and comparison with other fossils, my best guess of what this fossil could be is a possible species of Exogyra. All other similar looking fossils in my small collection do not have any color like this specimen and I haven’t seen any other fossils this well preserved (if that is the original color of the shell). I don’t know too much about the evolution of bivalves but another guess is it a “newer” species from a time period other than Cretaceous and is younger or not as weathered. 

 

I am open to any other suggestions, questions and especially confirmation of what the little critter was!

[grandpa]

Ilymatogyra arietina (formerly Exogyra a.)