mussel vs mytilid

mussel and mytilid both are nouns.

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

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

mussel: marine or freshwater bivalve mollusk that lives attached to rocks etc.
mytilid: marine bivalve mollusk having a dark elongated shell; live attached to solid objects especially in intertidal zones

Other hyponyms of mussel include marine mussel, freshwater clam, freshwater mussel.

mussel (noun)	mytilid (noun)
marine or freshwater bivalve mollusk that lives attached to rocks etc.	marine bivalve mollusk having a dark elongated shell; live attached to solid objects especially in intertidal zones
black marine bivalves usually steamed in wine