Here are three photographs of what I presume are a type of clam shell I found on the shore of Holden Beach, NC.
According to The Shell Book, Atlantic, Gulf, Carribean, this shell most strongly favors the listing on p.52. This shell is common to North Carolina, known as the "Chalky Buttercup Shell", under Lucines and Bittersweets.
If this is the correct identification, then this particular shell's latin name is Anodontia philippiana, with measurements of 2-4" common from NC to Eastern Florida, Cuba and Bermuda.
Your "chalky buttercup shell" is probably an elegant dosinia (Dosinia elegans) which is pretty common on NC beaches. Lots of times, you can find them still attached at the hinge. They grow to about 4 inches.
Terri K. Hathaway
Marine Education Specialist
North Carolina Sea Grant