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 scales spinulate over a considerable portion of the upper side. The chief difference between the Baltic and the North Sea is the reduced salinity of the former, so that it might be supposed that fresher water caused the greater development of the dermal skeleton. On the other hand, a species or geographical variety of the Plaice, whose proper is _P. glacialis_, is found on the Arctic coasts of Asia and America, on both sides of the extreme North Pacific, and on the east coast of North America. In this form the bony tubercles on the head in the Plaice are replaced by a continuous rough osseous ridge, and the scales are as much spinulated as in the Plaice of the Baltic. On the east coast of North America the males in this form are more spinulated than the females; on the Alaskan coast, and apparently the Arctic coast, the females are spinulated, and the sexual difference in this respect is slight or absent. Lower salinity cannot be the cause of greater spinulation in this case, and thus it might be suggested that the condition was due to lower temperature. But we do not find that northern or Arctic species of fish in general have the scales more developed than southern species.