mytilidEtymologyBorrowed from both translingual Mytilida and Mytilidae.mytilid means any bivalve mollusc of the order Mytilida; the mussels. Lexicurio rates it Sui generis — a strength score of 87 out of 100.nounAny bivalve mollusc of the order Mytilida; the mussels.Any mussel of the family Mytilidae.