248 ON OOLOUB-VABIATIOJf IN FtAT-FlSHES. [Mar. 6, 



symmetry witb each other, are related to each other in such a way 

 that they may undergo similar variations simultaneously. Upon 

 the deductions from this principle I have dwelt elsewhere. 



Several forms of abnormal pigmentation upon the " blind " side 

 of Flat-fishes are of course famihar, but of the particular variation 

 here seen I have met with no other case. In a recent paper, how- 

 ever, Cunningham ' makes allusion to cases apparently of this 

 nature, saying that they are frequent in the Brill. So far as I 

 know, the occurrence is not mentioned by the other authors who 

 have treated of the colour-variations of flat-fishes. 



The other specimen is one to which I lately made reference in 

 writing on the subject of pigmentation in the blind sides of Flat- 

 fishes ". The description that I gave was very brief and not quite 

 correct, and I take this opportunity of amphfying and correcting 

 it. It may appropriately be considered here inasmuch as it 

 also illustrates the influence of Symmetry in determining the 

 manner of occurrence of Variation, though in a way different from 

 that seen in the Brill described above. The fish is a Plaice {Pleu- 

 ronectes platessa), also received from Mr. Dunn. Its fins, eyes, &c. 

 were normal. The posterior half of the "blind" side was fully 

 pigmented, the pigmented area being sharply limited at a sinuous 

 line slightly behind the level of greatest width. This pigmented 

 area was of the same colour as the skin of the upper surface, and, 

 like it, bore spots of a full orange colour. Of these spots there 

 were, in all, thirteen — eight being on the body, three on the dorsal 

 fin, and two on the anal. The interest of the case lies in the fact 

 that by passing pins vertically through the body it was proved that 

 the centres of nine of these spots coincided exactly with the centres 

 of spots on the upperside. Four of these coincident spots were 

 ventral to the lateral line, two being on the body and two on the 

 anal fin. One large spot was upon the lateral line. Three were 

 upon the dorsal fin, and one, a large spot, was also upon the body, 

 just anterior to the base of the caudal fin. There was one spot 

 over the muscles of the dorsal fin which very nearly corresponded 

 with a similar spot on the upperside. 



In the same region were two more spots on the lower side that 

 were each represented on the upper side, but they were not in 

 correspondence with their representatives, but alternated with 

 them. One large spot on the lower side, ventral to the lateral line, 

 anterior to the base of the caudal fin, was wholly unrepresented 

 on the upper side. 



The manner of occurrence of this variation proves that, though 

 in a normal flat-fish there is a great dissimilarity between the 

 coloration of the upper and lower sides, yet that, when the lower 

 side assumes the characters of the upper, it may do so in such a 



^ Cunningham, J. T., Phil. Trans. 1894, ckxiciv. B, p. 807. 



^ ' Materials for the Study of Variation,' 1894, p. 467. The account there 

 given contains a misprint. For " of these, 13 spots on body and fins coincided " 

 read " of these 13 spots on body and fins, 9 coincided." 



