The adaptive character and possible course of evolution—following permanent byssal attachment—of the heteromyarian form in the Bivalvia is considered in relation to the eulamellibranch Dreissenacea and the filibranch Mytilacea. In both it assumes the extreme condition of ventral (anterior) flattening representing a high degree of adaptation for epifaunal life. The presence of a shell shelf in the umbonal regions of Dreissena and Septifer is associated with the greatest degree of ventral flattening; it permits retention of the anterior adductor and its attachment to parallel surfaces. Enlargement of the posterior regions of the visceropedal mass and the posterior territory of the mantle/shell involves formation of an elongated opisthodetic ligament. That of the Dreissenacea, here initially described, is of unique complication. In primitive infaunal isomyarians an organ of locomotion through soft substrates, in these heteromyarians the foot becomes essentially concerned with byssal attachment with which much the greatest part of the small anterior and large posterior retractors becomes exclusively concerned. Although strikingly similar to the Mytilacea, the Dreissenacea which now inhabit slowly moving or still, silt-laden freshwaters would seem most probably to have evolved under very different marine conditions prior to their relatively recent migration into freshwaters.