Synonyms for Hard-shell Clam (same or very similar meaning)
WordNet sense 1 (Atlantic coast round clams with hard shells; large clams usually used for chowders or other clam dishes):
quahog, quahaug, round clamWordNet sense 2 (an edible American clam; the heavy shells were used as money by some American Indians):
hard clamFrom the ODE community, based on WordNetadd/edit