26 CLASSIFICATION AND ADAPTATION 



the body, with spinules on the projecting edges, 

 making the skin rough ; lateral line with a semi- 

 circular curve above the pectoral fin. 



P. microcephalus, the Lemon-dab : scales small, 

 smooth, and imbedded ; skin slim}^ head and mouth 

 very small, colour yellowish brown with large round 

 darker marks. 



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 

 akeady, 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 dis- 

 tribution 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 



