These little clams ('baby clams' to Americans) are essential to that south Italian classic, spaghetti or linguine in clam sauce—spaghetti alle vongole. At markets you'll see vongole veraci— 'true clams' (carpet shell clams in English). If the shells of these are dark it's probably a new arrival, a related species from the Philippines that is now farmed commercially in Italy. They're very tasty, and the government allows them to be called vongole veraci too.
The more common vongole gialle, or yellow carpet shell clams, are nearly as good.