hard clam vs hard-shell clam

hard clam and hard-shell clam both are nouns.

Word	Noun	Adjective	Verb	Adverb
hard clam	Yes	No	No	No
hard-shell clam	Yes	No	No	No

As nouns, hard clam and hard-shell clam are synonyms defined as:

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

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

hard clam (noun)	hard-shell 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