The sea clam, more properly known as the Atlantic Surf clam (Spisula solidissima to malacologists), is a big critter: we keep only clams that are as big as the palm of a grown man's hand. After shucking, about 2/3 of the clam is edible, so you can get a good meal from a couple of them.
Sea clams have a large foot that provides a lot of meat. That's what was used by Howard Johnson when they popularized clam strips.
Sea clams are mighty fine eating. They have a naturally sweet flavor that makes great New England-style chowder, especially with summery herbs. I also enjoy it in a Portuguese Pork Alentejana.
The meat is found mostly in a large foot and two adductor muscles, plus a few odd bits. While shelling clams, it is tempting to eat a couple of those adductor muscles, and that's a good thing.
I used sea clams to make an old Cape Cod Sea Clam Pie from Suzanne Cary Gruver's Cape Cod Cook Book. It was delicious, and a surprise hit at work, where people were carving off slices and eating it cold because it tasted too good to wait for the microwave!
Here's a nice post about sea clamming in Wellfleet by Dick Nicholson.