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Show 438 MR. E. W. L HOLT-STUDIES IN [May 1, We have therefore, in the ambicolorate but otherwise normal Brill, a reproduction on the blind side of those markings which are most conspicuous in the young of the same and of most other species of Pleuronectids, and wdiich are characteristic, of course on the ocular side, of the smallest species of the genus Rhombus and its immediate allies. I wish to draw especial attention to these facts, as I consider that they have a distinct bearing on the interpretation of the phenomenon of ambicoloration. Mr. Cunningham, in the Boyal Society memoir so frequently referred to, establishes the fact that pigment can be produced by the action of light on the colourless under surface of an already metamorphosed flat-fish, noting, at the same time, the great variation which exists in the susceptibility of individuals to this treatment. The authors reject, however, the hypothesis that the ambicolorate condition so commonly met with can also be explained as due to the action of light, since there is " not the slightest evidence at present that these abnormal specimens have been exposed to abnormal conditions, or have had abnormal habits of life," and, for m y own part, I most certainly agree with them in the main. There is, however, one colour abnormality, so common as perhaps hardly to merit such a designation, which I think is probably due to the action of light. This is the presence of more or less pigment on the blind side of the median fins. It m a y be present merely in the form of irregular dull streaks or splashes, as in the caudal fin of the Sole, which forms the subject of this paper, or it may be developed to such an extent that practically the whole of the marginal fins are coloured. This last condition is exemplified by a Dab (PI. limanda) in m y possession, but the pigment of the blind is as brilliant as that of the ocular side, and there are a few bright spots on the blind side of the body as well. I find no difficulty in attributing such a dull and partial pigmentation as is present on the caudal fin of the Sole to the action of light through the transparent tissues of the fin, but the brilliant coloration of the Dab's fins, associated, as it is, with a partial, if very slight, development of colour on the non-transparent body, seems to require some further explanation. Messrs. Cunningham and M a c M u n n find a difficulty in accepting reversion or atavism as an explanation of the ambicolorate condition, in that the hypothetical vertically swimming ancestor of the Flat-fish must have " had an unpigmented white or silvery ventral surface, as other symmetrical fishes have," whereas completely ambicolorate flat-fish are uniformly pigmented all over. The difficulty certainly arises if w e assume that the ancestor really was paler on the ventral region than elsewhere ; but is it not equally reasonable to assume a stage of evolution in which the fish resembled such forms as Platax or Dascgllus, to take instances from families widely separated from each other by systematists ? Both forms have high compressed bodies, and in some species, at any rate, in both genera the ventral region is as deeply pigmented as the dorsal. Even in the John Dory (Zeus faber), in which the |