OK, that one doesn't hit the aquarium market very often, but if it looks like that, it's probably a dark variation of Cypraea arabica. About 1.5 to 2 inches long? They are easier to keep in a tank than most species. They will scrape the algae off your tank walls and off your rocks, but you may not have much for them to eat in your tank. You can supplementally feed them wilted leaf lettuce and clams on the half shell like you buy at the grocery store, but it may take a while for them to get used to eating things they don't normally eat. The crabs will probably eat what they don't. Some Cypraea arabica are very dark, almost black, but most look like this:
Marine Fish Picture Gallery - Arabian Cowry picture
Cheers,
Don