round clam vs hard clam

round clam and hard clam both are nouns.

Word	Noun	Adjective	Verb	Adverb
round clam	Yes	No	No	No
hard clam	Yes	No	No	No

As nouns, round clam and hard clam are synonyms defined as:

round clam and hard clam: an edible American clam; the heavy shells were used as money by some American Indians

Other synonyms of round clam include hard-shell clam, Mercenaria mercenaria, quahaug, quahog, Venus mercenaria.

round clam (noun)	hard clam (noun)
an edible American clam; the heavy shells were used as money by some American Indians	an edible American clam; the heavy shells were used as money by some American Indians
Atlantic coast round clams with hard shells; large clams usually used for chowders or other clam dishes