Sinistral Shell

My daughter gave me this pretty shell that she found on the beach in Jacksonville, Florida. She didn't realize that there was a little hermit crab living inside of it. The shell is very small, about two inches across. I tried to identify the kind of seashell using Google, but couldn't find an exact match. I did notice that it is extremely unusual, though, because it is a left-opening (sinistral) shell. Most all seashells are right opening. 
This is a focus stack.