hard clam vs littleneck

hard clam and littleneck both are nouns.

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

As nouns, littleneck is a hyponym of hard clam; that is, littleneck is a word with a more specific, narrower meaning than hard clam:

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

Other hyponyms of hard clam include littleneck clam, cherrystone, cherrystone clam.

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