CLASSIFICATION AND ADAPTATION 27 



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 develop- 

 ment 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 name 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 con- 

 dition was due to lower temperature. But we do 



