I think just a regional use of the word quahog. Technically all hard clams (mercinaria mercinaria) are quahogs although in new england quahogs are the largest followed by cherrystones, top necks and then littlenecks. On Long Island we start with chowders, cherries, tops and necks. Then there are ocean quahogs which look alike but the shells are very dark and they don't taste very good, you often see them in supermarkets. And don't forget florida maricultured hard clams from ceder key, sea or surf (think Doxee) clams and soft clams which are steamers on long island and belly or pisser clams in new england. Wow, I'm suddenly feeling very clammy this morning.