It's a kind of shellfish, which has become rare apparently, harvested in Italian waters since the Romans. They're protected in Liguria and maybe elsewhere. They're probably completely unobtainable in the US, and not easy to get in Italy. Maybe Siduri has something to offer.
Elizabeth David has an interesting description in her imaginatively named book, "Italian Food," mostly quoted from an 18th Century novelist/travel-food writer/physician/naturalist, Tobias Smollet. They're supposedly similar to mussels in taste and texture, and look something like dates (datteri). Smollet wasn't impressed and David was oddly reticent in offering her opinion.
If you don't know, Elizabeth David was one of the great all time cookbook writers -- even though her cookbooks meander off the subject frequently -- and charmingly. She co-wrote a book with Julia Child, and IIRC Child said David was one of her favorite writers and had a lot to do with Child taking up food as a profession. If you haven't read David, you should. Tobias Smollet wasn't half bad either if you like picaresque novels with some rather creative set-ups. He was early to "high concept," but handled plotting better than many. His "Travels through Italy and France" (I think that's what it's called) was kind of bittersweet. It was a trip he and his wife took to soothe the open wound left by the untimely death of their daughter. I read it about thirty years ago, so am very hazy; it reminded me a little of Pepys, but a bit more self-conscious.