“Big bellies,” “piss clams,” “Ipswich,” “steamers,” “soft shells”–these are all names for the same type of clam, says our resident marine biologist, FlyFish. The species is Mya arenaria; they’re the ones most often used for frying. They’re almost always what you’re eating when you’re eating whole fried clams. Typically, the only other kind of clam that gets fried is surf clam, which gets cut up and sold as “fried clam strips.”
The best soft shell clams are the ones with the biggest bellies. Luckily, says FlyFish, a lot of better claim joints will select only the bigger-bellied clams. (They’re wonderful simply steamed too.)
Here’s a clammy web site.
Big belly fried clams