A subclass of bivalve mollusks characterized by a foot with a sole that is divided sagittally and longitudinally and has papillate margins. The foot is without a true byssus gland, although a nonhomologous byssal gland is frequently present in the heel. The byssal gland does not produce threads and has a structure totally different from the byssus gland of the subclass Lamellibranchia. The gill filaments, which are not reflected, are usually platelike with abfrontal cilia. Thus the gills (etenidia) resemble those of aspidobranch gastropods rather than those of the lamellibranch bivalves. Digestion is entirely extracellular.