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 fiesus), 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 ever. 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 
