Subclass of bivalves (Bivalvia), which have heterodont dentition consisting of a hinge plate with distinct cardinal teeth below the umbo, and lateral teeth posterior to the cardinals. Some forms may be desmodont. The ligament is opisthodetic. Shell shape varies according to the mode of life of the organism, most having a crossed-lamellar, aragonitic shell structure. The pallial line is entire, or with a sinus. Heterodonta are mainly infaunal siphon-feeders, the siphons being well developed. They first appeared in the Middle Ordovician.