What does "mytilid" mean?

Any bivalve mollusc of the order Mytilida; the mussels.

Mytilid

Common synonyms include: bivalve mollusk, edible mussel, marine mussel, mussel, mussel like traits, mussel related.

noun

noun ·Rare ·Advanced level

Noun

1
Any bivalve mollusc of the order Mytilida; the mussels.
2
marine bivalve mollusk having a dark elongated shell; live attached to solid objects especially in intertidal zones wordnet
3
Any mussel of the family Mytilidae.

Borrowed from both translingual Mytilida and Mytilidae.