Flatfish entire order was John Roxborough Norman's "Monograph of the Flatfishes" published in 1934. New species are described with some regularity and undescribed species likely remain. Hybrids are well known in flatfishes. The Pleuronectidae, of marine fishes, have the largest number of reported hybrids. Two of the most famous intergeneric hybrids are between the European plaice ("Pleuronectes platessa") and European flounder ("Platichthys flesus") in the Baltic Sea, and between the English sole ("Parophrys vetulus") and starry flounder ("Platichthys stellatus") in Puget Sound. The offspring of the latter species pair is popularly known as the hybrid sole and was initially believed to
Flatfish around "ponds" of sulphur at hydrothermal vents on the seafloor. No other flatfish is known from hydrothermal vents. Many species will enter brackish or fresh water, and a smaller number of soles (families Achiridae and Soleidae) and tonguefish (Cynoglossidae) are entirely restricted to fresh water. The most obvious characteristic of the flatfish is its asymmetry, with both eyes lying on the same side of the head in the adult fish. In some families, the eyes are usually on the right side of the body (dextral or right-eyed flatfish), and in others, they are usually on the left (sinistral or left-eyed