bivalve vs ark shell

bivalve and ark shell both are nouns.

Word	Noun	Adjective	Verb	Adverb
bivalve	Yes	Yes	No	No
ark shell	Yes	No	No	No

As nouns, ark shell is a hyponym of bivalve; that is, ark shell is a word with a more specific, narrower meaning than bivalve:

bivalve: marine or freshwater mollusks having a soft body with platelike gills enclosed within two shells hinged together
ark shell: marine bivalve mollusk having a heavy toothed shell with a deep boat-like inner surface

Other hyponyms of bivalve include clam, cockle, oyster, blood clam, mussel, escallop, scallop, scollop, piddock.

bivalve (noun)	ark shell (noun)
marine or freshwater mollusks having a soft body with platelike gills enclosed within two shells hinged together	marine bivalve mollusk having a heavy toothed shell with a deep boat-like inner surface

bivalve (adjective)	ark shell (adjective)
used of mollusks having two shells (as clams etc.)