pelecypod vs oyster

pelecypod and oyster both are nouns.

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

As nouns, oyster is a hyponym of pelecypod; that is, oyster 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
oyster: marine mollusks having a rough irregular shell; found on the sea bed mostly in coastal waters

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

pelecypod (noun)	oyster (noun)
marine or freshwater mollusks having a soft body with platelike gills enclosed within two shells hinged together	marine mollusks having a rough irregular shell; found on the sea bed mostly in coastal waters
	edible body of any of numerous oysters
	a small muscle on each side of the back of a fowl

pelecypod (adjective)	oyster (adjective)
bivalve

pelecypod (verb)	oyster (verb)
	gather oysters, dig oysters