We are truly blessed in the UK to be surrounded by a superb array of flat fish, which - barring Halibut - are not found in the rest of the world. You will find various soles in American, Indian and African waters, but they tend to be of lesser quality.
All flat fish are born round, then as they grow, the eyes move to either the left or right side of the fish so they can see all around when lying on the sea bed. Left-eye fish are called senistral (Turbot, Brill & Megrim) and right-eye are called dextral (all others). They all have dark skin on the 'eye side' and white on the 'blind side' except Greenland Halibut which is dark both sides.
Fillets from flat fish have fewer bones compared to round fish, and the larger species (Halibut, Turbot and Brill) also provide excellent boneless supremes from fish over 3kg.