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Show 248 ON COLOUR-VARIATION IN FLAT-FISHES. [Mar. 6, symmetry with 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 familiar, but of the particular variation here seen I have met with no other case. In a recent paper, however, 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 w ho 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 Flatfishes 2. The description that I gave was very brief and not quite correct, and I take this opportunity of amplifying 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 tbe 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 1 Cunningham, J. T., Phil. Trans. 1894, clxxxiv. B, p. 807. 2 ' 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." |