lamellibranch vs pelecypodous

lamellibranch is a noun but pelecypodous is not a noun.

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

As adjectives, lamellibranch and pelecypodous are synonyms defined as:

Other synonyms of lamellibranch include pelecypod.

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

lamellibranch (adjective)	pelecypodous (adjective)
bivalve	bivalve