pelecypodous vs lamellibranch

pelecypodous is not a noun while lamellibranch is a noun.

Word	Noun	Adjective	Verb	Adverb
pelecypodous	No	Yes	No	No
lamellibranch	Yes	Yes	No	No

As adjectives, pelecypodous and lamellibranch are synonyms defined as:

Other synonyms of pelecypodous include pelecypod.

pelecypodous (noun)	lamellibranch (noun)
	marine or freshwater mollusks having a soft body with platelike gills enclosed within two shells hinged together

pelecypodous (adjective)	lamellibranch (adjective)
bivalve	bivalve