X THE COLOURS OF FISHES 217 



in part a type of coloration in all respects similar 

 to that of the upper surface, the similarity being 

 carried out into the details of spots and markings. 

 Such forms are called " double " or " ambicolorate." 

 This peculiarity has been described in the turbot 

 {Rhombus maximus), the brill {R. Icsvis), the flounder 

 {Pleuronectes flesus), the plaice {P. platessa), the 

 merry sole {P. microcephalus), and perhaps occurs 

 in others. An exceedingly interesting point about 

 the variation is that in the turbot, the brill, and the 

 flounder it is when marked always associated with 

 a peculiar malformation of the head, due to a grow- 

 ing forward of the dorsal fin. In the plaice it is 

 occasionally so associated, in the merry sole rarely 

 if even In the turbot the correlation is so exact 

 that, according to Mr. Cunningham, " if we draw an 

 imaginary line through the preopercular bone in the 

 turbot, pigmentation may extend over the whole 

 of the lower surface behind this line without any 

 structural malformation being present, but when 

 pigment is also present on the lower side in front of 

 this line the characteristic structural malformation 

 occurs also ; on the other hand, the structural mal- 

 formation has never been observed in any speci- 

 mens in which the lower side was unpigmented or 

 pigmented to a less extent than that defined above." 

 The meaning of the structural malformation is not 

 quite clear, though it has been suggested that it is 

 due to a delay in the shifting of the eye from the 

 blind side ; to explain its association with the pig- 

 mentation of the lower surface, it has been further 

 suggested that the delay allows a " continuation of 

 the power of receiving visual sensations from this 



