Symmorium is an extinct symmoriiform cartilaginous fish from the Devonian and Carboniferous of the United States (Illinois) and Russia. The type species, Symmorium reniforme, was named by Edward Drinker Cope in 1893, with other species assigned to the genus having since been reclassified into other genera such as Petalodus. Symmorium bears close similarity in size and appearance to Stethacanthus but lacks the "spine-brush complex" in place of the first dorsal fin. Some paleontologists think that the two forms represented the males and females of related species, while other scientists think they were distinct genera.