unionoid
/ˈjuːnɪənɔɪd/Zoology
adjective
Of or relating to the family Unionidae of freshwater mussels, or the order Unionoida in which they are included; unioniform.
noun
A mollusc of the family Unionidae or order Unionoida; a freshwater mussel.
Origin
Late 19th century; earliest use found in Henry Nicholson (1844–1899), biologist and geologist. From scientific Latin Unio unio + -oid, with insertion of -n- after classical Latin ūniōn-, ūniō, etymon of the first element.