_P. cynoglossus,_ the Witch or Pole-dab: head and mouth smaller than in
the Plaice, eyes rather larger; scales all alike and uniformly
distributed, slightly spinulate on upper side, smooth on the lower;
blister-like cavities beneath the skin of the head on the lower side.

With regard to the generic characters, it is difficult to give any reason
why the mouth should be at the end of the head instead of behind the apex
of the snout as in the genus _Solea,_ but, as we have seen already, the
small size of the mouth and the greater development of teeth on the lower
side are adapted to the food and mode of feeding. It is impossible to say
why one genus of Flat-fishes should have the right side uppermost and
others, _e.g._ Sole and Turbot, the left; it would almost seem to have
been a matter of chance at the commencement of the evolution: reversed
specimens occur as variations in most of the species.

When we consider the specific differences, we find very definite
characters in the structure and distribution of the scales, and no
evidence has yet been discovered that these differences are related to
external conditions. There are, of course, slight differences in habits
and habitat, but no constant relation between these and the structural
differences of the scales. Plaice and Dab are taken together on the same
ground, and nothing has been discovered to indicate that the spinulate
scales of the Dab are adapted to one peculiarity in habits or conditions,
the spineless scales of the Plaice to another. In comparing certain
geographical races of Plaice and Flounder the facts seem to suggest that
differences of habitat may have something to do with the development of
the scales. In the Baltic the Flounders are as large as those on our own
coasts, but the thorny tubercles are much more developed, nearly the whole
of the upper surface being covered with them. The Plaice, on the other
hand, are smaller than those of the North Sea, and the _males_ have the