littleneck vs hard clam

littleneck and hard clam both are nouns.

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

As nouns, hard clam is a hypernym of littleneck; that is, hard clam is a word with a broader meaning than littleneck:

littleneck: a young quahog
hard clam: an edible American clam; the heavy shells were used as money by some American Indians

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

littleneck (noun)	hard clam (noun)
a young quahog	an edible American clam; the heavy shells were used as money by some American Indians
a quahog when young and small; usually eaten raw; an important food popular in New York