heteromyaria

from The Century Dictionary.

• A group of acephalous conchiferous mollusks, including the Mytilidæ, or mussels and related forms: distinguished from Dimyaria and Monomyaria.

from the GNU version of the Collaborative International Dictionary of English.

• noun plural (Zoöl.) A division of bivalve shells, including the marine mussels, in which the two adductor muscles are very unequal. See dreissena, and Illust. under byssus.