"As birds are seen to occupy very different situations, some obtaining their food on the ground, others on trees, and not a few at various degrees of elevation in the air, so are fishes destined to reside in different depths of water. The flat-fishes and the various species of skate are, by their depressed form of body, admirably adapted to inhabit the lowest position, where they occupy the least space among their kindred fishes."
"Preferring sandy or muddy shores, the place of the flat-fish is close to the ground; where, hiding their bodies horizontally in the loose soil at the bottom, with the head only slightly elevated, an eye on the under side of the head would be useless; but as both eyes are placed on the upper surface, an extensive range of view is afforded in those various directions in which they may either endeavour to find suitable food or avoid dangerous enemies. Light, one great cause of colour, strikes on the upper surface only; the under surface, like that of most other fishes, remains perfectly colourless. Having little or no means of defence, had their colour been placed only above the lateral line on each side, in whatever position they moved their piebald appearance would have rendered them conspicuous objects to all their enemies. When near the ground, they swim slowly, maintaining their horizontal position; and the smaller pectoral and ventral fins, on the under side, are advantageous where there is so much less room for their action than with the larger fins that are above. When suddenly disturbed, they sometimes make a rapid shoot, changing their position from horizontal to vertical; and, if the observer happens to be opposite the white side, they may be seen to pass with the rapidity and flash of a meteor. Soon, however, they sink down again, resuming their previous motionless horizontal position, and are then distinguished with difficulty, owing to their great similarity in colour to the surface on which they rest."
The number of species of the flat-fishes diminishes as the degrees of northern latitude increase. In this country we have twenty-three species; at the parallel of Jutland there are thirteen; on the coast of Norway they are reduced to ten; in Iceland the number is but five, and in Greenland only three.