pelecypod vs mussel

pelecypod and mussel both are nouns.

Word	Noun	Adjective	Verb	Adverb
pelecypod	Yes	Yes	No	No
mussel	Yes	No	No	No

As nouns, mussel is a hyponym of pelecypod; that is, mussel is a word with a more specific, narrower meaning than pelecypod:

pelecypod: marine or freshwater mollusks having a soft body with platelike gills enclosed within two shells hinged together
mussel: marine or freshwater bivalve mollusk that lives attached to rocks etc.

Other hyponyms of pelecypod include clam, cockle, oyster, ark shell, blood clam, escallop, scallop, scollop, piddock.

pelecypod (noun)	mussel (noun)
marine or freshwater mollusks having a soft body with platelike gills enclosed within two shells hinged together	marine or freshwater bivalve mollusk that lives attached to rocks etc.
	black marine bivalves usually steamed in wine

pelecypod (adjective)	mussel (adjective)
bivalve