Most diverse flatfish family

The taxonomic order Pleuronectiformes, constituting the flatfishes, is of global oceanic distribution and contains more than 700 species. Its most diverse taxonomic family, containing more species than any other within this order of fishes, is Soleidae. This family contains approximately 185 species (depending upon how certain species are split or lumped taxonomically), classified within 30–32 genera, and they constitute the true soles. Their physical characteristics are: both eyes are on the right side of the head; the preoperculum's margin is completely concealed (covered by skin and scales); the dorsal fin originates above or before the eyes; the pectoral fins may be present or absent, but if present they lack fin spines; the dorsal and anal fins can be either free from or united with the caudal fin; and the pelvic fins are free from the anal fin.

Several other flatfishes are commonly referred to as soles but are not members of Soleidae. These include the familiar lemon sole and Pacific dover sole, which belong to the family Pleuronectidae and taxonomically speaking are right-eye flounders; the tongue soles (aka tonguefishes), belonging to the family Cynoglossidae; and the American soles, belonging to the family Achiridae (although some researchers subsume this family within Soleidae).