datteri di mare
They're a prized delicacy, but you won't see many of them. Or at least you shouldn't. Date mussels, so called for their size and shape, spent twenty years or more literally eating through rock to build a home. For decades, the Italians used little pneumatic jackhammers to get them out, with disastrous results for the shore habitats. Gathering datteri has been illegal since 2007.
If you see any, they may be illegal, taken by organized crime gangs, or they may be equally illegal Chinese imports. Real datteri always have a reddish tint; the Chinese cousins are strictly greyish-black.
Don't give up hope. Recently the Pugliesi have had some success raising datteri in the Gulf of Manfredonia, and they've already put a few on the market.
Besides datteri, they're also called dàtoli, lettere, peverone, cannulicchi, tarle and no doubt many other dialect words.