Order:	Pleuronectiformes
Family:	Pleuronectidae
Genus and species:	Platichthys flesus

William Yarrell (1836) in "A History of British Fishes":

THE FLOUNDER.

FLOOK, Merret. -- MAYOCK FLEUKE, Edinb. -- BUTT.

Platessa	flesus,	Flounder,	Flem. Brit. An. p. 198, sp. 104.
"	"	Le Flet,	Cuvier, Règne An. t. ii. p. 339.
Pleuronectes	fluviatilis,	Fluke,	Willughby, p. 97, F. 4.
"	flesus	"	Linnĉus. Bloch, pt. ii. pl. 44 & 50.
"	"	Flounder	Penn. Brit. Zool. vol. iii. p. 305.
"	"	"	Don. Brit. Fish. pl. 94.

THE FLOUNDER is one of the most common of the Flatfish, and is found in the sea and near the mouths of large rivers all round our coast, being more particularly abundant where the bottom is soft, whether of sand, clay, or mud. All the bays, creeks, and inlets of Orkney produce it, according to Mr. Low; and it is taken in abundance in different parts of Scotland, where it is called Fluke and Mayock Fleuke, - a term having reference to the flattened form of the fish. It is common at Berwick and Yarmouth, at which latter place it is called a Butt - a northern term ; and those Flounders that are caught in the extensive backwaters behind Yarmouth, where there is a considerable deposit of mud, are in consequence so dark in colour as to be distinguished, from the lighter coloured ones caught on the sands of the sea, by the name of Black Butts. This similarity in colour between certain fishes and the bottom upon which they are found has been already referred to as affording security to the defenceless from the attacks of their enemies, and exhibits a beautiful instance of the design employed for the preservation of species. In Sweden, according to Linnĉus, this fish is called Flundra, from which our word Flounder is probably derived, and is said to refer to its manner of swimming when close to the ground.
The Flounder lives and thrives whether stationary in the sea, the brackish water, or the fresh water. In the Thames it is taken as high up as Teddington and Sunbury: Mr. Jesse mentions having seen the Flounder pursue Minnows with great eagerness into the shallows where the Mole runs into the Thames at Hampton Court. This species is caught in considerable quantities from Deptford to Richmond by Thames fishermen, who, with the assistance of an apprentice, use a net of a particular sort, called a tuck-net, or tuck-sean. One end of this net is fixed for a short time by an anchor or grapple, and its situation marked by a floating buoy ; the boat is then rowed, or rather sculled, by the apprentice in a circle, the fisherman near the stern handing out and clearing the net : when the circle is completed and a space inclosed, the net is hauled in near the starting point in a direction across the fixed end.
Flounders ascend rivers generally. Colonel Montagu says they are found up the Avon within three miles of Bath. They have been successfully transferred to fresh-water ponds: being long-lived out of water, the carriage from one place to another is a matter of very little difficulty. Along our southern shore the Flounder is very common ; and it occurs on the Irish coast from Cork up the eastern side to Antrim, and thence northward and westward to Donegal.
The Flounder feeds upon aquatic insects, worms, and small fishes, and has been known to acquire the weight of four pounds, but is not usually seen near so large. It spawns in February or March, and the young Flounders may be seen alive by the end of April. Varieties of the Flounder occur much more commonly than those of any other species of Flatfish. I have before me, while now writing, specimens without any colour on either side ; specimens coloured on both sides ; and specimens with both eyes and the whole of the colour on the left side instead of the right. Those without any dark colour on either side are albino varieties, through the transparent skins of which the colour of the bloodvessels and muscles has suggested the trivial names of rosea and carnaria to the authors who considered them species. The Pl. passer, figured by Bloch, pt. ii. pl. 50, is certainly only a reversed Flounder, having the eyes and the colour on the left side ; - a variety so common, that it is scarcely possible to examine a peck-measure of Flounders without finding one or more reversed specimens. One of the most remarkable specific distinctions of the Flounder, the series of denticulated tubercles placed between the rays of the fins along the dorsal and abdominal lines, is distinctly figured in both Bloch's plates as quoted, pt. ii. plates 44 and 50.
The length of the head is to the whole length of the fish as one to four ; the greatest width of the body, without the fins, is to the whole length of the fish as one to three : the mouth small; the teeth in one row in each jaw, small and numerous ; the upper eye nearly over the lower ; the lateral line but very slightly curved over the pectoral fin, and marked with numerous rough stellated tubercles at its commencement, some more of which are arranged in two lines, one above, the other below the lateral line throughout its course : the body smooth ; the scales small ; the dorsal and abdominal lines armed with a series of denticulated tubercles, one in each space, between the rays, and alternating with them ; the dorsal fin extends from the eye almost to the tail ; the ventral fin is placed a little farther back than in the Plaice, under the margin of the operculum ; the anal fin, preceded by a spine directed forwards, also commences farther back ; both dorsal and anal fins terminate on the same plane ; the fleshy portion of the tail narrow, its rays elongated, and almost square at the end. The fin-rays in number are --

D. 55 : P. 11 : V. 6 : A. 42 : C. 14.

The colour of this species is variable, the shades of brown depending on the nature of the ground from which the fish was taken, but generally mottled with darker brown; the fins light brown, occasionally varied with patches of darker brown, but generally lighter than the body. Examples sometimes occur with a few indistinct reddish spots on the upper surface ; but the roughness of the lateral line in the Flounder, and its smoothness in the Plaice, is a distinguishing character in these two species, however similar they may happen to be in colour or size.

Frank Buckland (1880) in "Natural History of British Fishes":

THE FLOUNDER.

German: Die Flunder, Sandbutt. Danish: Skrubbe. Dutch: Bot. French: Le Flet.

IT inhabits every part of the British seas, and often frequents our rivers beyond the reach of the tide, thriving in salt, brackish, or fresh water. Now that the Thames is getting purer, the flounders are, I am glad to say, returning to the river above London Bridge. The flounder is much smaller than the plaice, it is also more elongated in form, and has a row of sharp, denticulated tubercles, which surround its sides, and are placed along the base of the dorsal and anal fins. These are specimens without any colour on either side, specimens coloured on both sides, and specimens with both eyes and the whole of the colour on the left side, instead of the right ; while white or Albino varieties, are not uncommon.

Alwyne Wheeler (1969) in "The Fishes of the British Isles and North West Europe":

Flounder

Platichthys flesus (Linnaeus, 1758)

NAMES Fr. Le Flet, Picaud; Du. Bot, Botje; Ge. Flunder; Da. Skrubbe; Nor. Skrubben.

IDENTIFICATION The mouth is terminal, relatively small and directed to the right of the eyes (reversed examples are very common). The head is moderate, rather more than a quarter of the body length. There are one or two bony knobs (but not a series) at the beginning of the lateral line. The lateral line is slightly curved above the pectoral fin. The body scales are small and smooth edged, but either side of the lateral line, and particularly between it and the pectoral fin, there are a few coarsely toothed scales, giving this region a rough feel. A single series of similar prickles along the bases of dorsal and anal fins are the most noticeable identifying feature. Fifty-two to sixty-seven dorsal rays; thirty-five to forty-six anal rays; thirty-six vertebrae. Grows to about 20 in (51 cm).
Colour: a dull brown, greyish or dull green on the eyed side, occasionally with light orange speckles (but never so bright as in the plaice). The blind side is a dead white and opaque.

BIOLOGY The flounder is well known as the only European flatfish to penetrate well into estuaries and to live in freshwater for short periods. In general, it is common in inshore waters within the 30 fathom (55 m) line, but may well be rather local in its distribution within a specific area. It belongs essentially to estuaries and is most common where water of low salinity is found; it is particularly common in the Baltic Sea, and also the southern and eastern coastal North Sea. Although frequently found in fresh water it will not spawn there and eventually returns to the sea. The flounder makes surprisingly distant migrations; distances of 120-200 miles have been recorded, although the average is not much more than 20 miles. On the other hand, many marked flounders have not moved significantly over a period of several years. Their distant migrations are primarily made in summer time, as are their entries into fresh water, and may be basically breeding migrations. Local movements in the sea also take place and are seasonal in their nature, inshore in early summer, returning to deeper water in winter.
Feeding takes place mainly during the warmer months of the year, and in mid-winter may virtually cease. The young fish below 3 in (8 cm) in length feed mainly on copepods, diatoms and (in brackish or fresh water) chironomid larvae. From 3-6 in (8-15 cm) they feed mainly on crustaceans, Corophium and amphipods in general, mysid shrimps, and particularly young shore crabs; they may eat small molluscs including young Macoma. Large flounders eat a large proportion of molluscs, in particular Macoma, cockles, mussels and Mya. They also feed heavily on polychaete worms in some areas, and crustaceans (brown shrimps, shore crabs, mysid shrimps and Corophium) everywhere. Flounders can be serious predators on cockle beds. Their typical feeding behaviour is to come into the intertidal zone with the incoming tide and feed heavily during the high tidal period.
An offshore migration precedes spawning mainly in March and April (February to June being the overall spawning period). Spawning takes place in water of 15-30 fathoms (27-54 m). The eggs are very small (0.95-1.02 mm), and hatch in about seven days at a temperature of 12°C (55°F). The eggs float near the surface, but slowly sink as development proceeds; hatching occurs at a length of 2.5-3 mm. They metamorphose at a length of ½-1ĵ in (15-30 mm), and fish beyond that length are usually bottom-living. These young fish also live close to the shore, and enter estuarine water. Growth thereafter is fast, but, of course, depends on temperature and food available; at the end of their first year they average 3 in (8 cm), at their second 5½ in (14 cm), at their third 7½ in (19 cm), at their fourth 9½ in (24 cm). Males reach sexual maturity at 4½ in (11 cm), females at 7 in (17 cm).
The flounder is an important food fish in parts of Europe (particularly in the Baltic and in Danish waters where the majority of the European catch is made - a total approaching 10,000 tons in 1963). The U.K. total is very small, mostly caught incidentally while fishing for other species. The flounder is slightly less palatable than the plaice. Flounders do, however, make a popular sport fish, as their availability in inshore waters, abundance and readiness to take bait all appeal to the sea angler. The British record rod-caught flounder (1956) weighed 5 lb 11½ oz (2.576 kg).

DISTRIBUTION

North to the White Sea; also western Mediterranean, Adriatic - P. flesus italicus (Günther, 1862) and Black Sea - P. flesus luscus (Pallas, 1814).