A direct quote from the man himself. Oddball from MFK writes:
These moon snails are fairly common. Unfortunately, this one is a mold (internal fossilization). The nacre and mantel didn't survive the fossilization process so, there's no way to accurately predict the species. Nice find.
Too bad we can't tell what it was for sure. Oh well, I'll keep my eyes on the ground and see if I can't stumble across another some day.