I think it is

Phylum: Mollusca (Cuvier)
Class: Pelecypoda
Order: Heterodonta
Superfamily: Hippuritacea
Family: Radiolitidae (Gray)
Genus: Durania
Species: austinensis

Geological Period: Cretaceous - 144 to 66.4 Ma
European Stage: Campanian - 84 to 74.5 Ma
Geological Group: Taylor - 80 to 71.5 Ma
Formation: Ozan - 81 to 77 Ma
Locale: N. Sulphur River area
County: Delta/Lamar
State: TX
Country: USA
 

Info based on online help from the Dallas Paleontological Society at http://www.dallaspaleo.org/paleo_is/pis_lst_specimens.asp?S=INFO&ID=14