RECAPITULATION 221 



other secondary sexual characters such as the antlers 

 of stags m one respect, namely, that since the Dab 

 (P. limanda), the 8ole, and other species of Solea, and 

 several other Pleuronectidae have what are called 

 ctenoid scales — that is, scales furnished with spines 

 on the posterior edge — and since the ordinary scales 

 of the Plaice are reduced, the spinulation of scales in 

 the mature male Plaice is not a new character but 

 the retention of a primitive character. Then the 

 question would remain why the scales in the mature 

 female and immature male have degenerated, or 

 rather why the primitive character develops only in 

 the mature stage of the male. 



There is one point in which this sexual dimorphism 

 in the Plaice appears to differ from typical cases, and 

 which suggests that the greater spinulation of scales 

 in the males has no function at all m the relations 

 of the sexes, and is therefore not subject to any 

 external stimulation. This point is the remarkable 

 way in which the degree of development of spiny 

 armature differs in different regions and in local 

 races, and seems to correspond to different climatic 

 conditions. Both Plaice and Flounders in the Baltic 

 are much more spiny than in the North Sea, although 

 in the Flounder no sexual difference in this respect 

 has been noted. On the east coast of North America 

 occurs P. glacialis, in which the scales of the male are 

 strongly spinulate and those of the female smooth. 

 On the coast of Alaska females of this species seem 

 to be more spinulate than elsewhere. The Flounder 

 does not occur in the Arctic, but on the west coast of 

 North America occurs a local form called P. slellaius, 

 scarcely distinct as a species, which has a strong 

 development of spiny tubercles all over the upper 



