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Show 440 MR. E. W. L. HOLT-STUDIES IN [May 1, spines in the smaller Norwegian examples is the more ancient. Since symmetry of dermal armature must have been as characteristic of the hypothetical Pleuronectid ancestor as symmetry of colour, the presence of spines or tubercles on both sides of ambicolorate flat-fish seems to m e an important evidence of reversion; further, the fact that when colour is present on the blind side of the Brill (and also of the Turbot, though the feature is less conspicuous) it commonly assumes the distribution characteristic of the young stages of nearly all Pleuronectids, and of the adult stages of the Topknots (the smallest and most strongly ciliated, and probably the most primitive of its allies), appears to strengthen the case considerably. A difficulty undoubtedly arises in the want of any known instance in which ambicoloration has been accompanied by a development of the muscles of the blind side equivalent to that which takes places on the coloured side, or of equal development of the pectoral fins. The dermal tubercles, however, even in Cyclopean Turbot, are never developed on the blind side to an extent corresponding to that of the ocular side, so that at best only a partial reversion can be argued for any feature which manifests itself in ambicolorate examples. In partially ambicolorate specimens, in which the pigment of the blind side is irregular, and not arranged in definite series of markings, as in the Brill, the theory of reversion would inculcate that the reversion to colour-activity and symmetry of scales is confined to certain areas of the derma. Total ambicoloration, as Messrs. Cunningham and M a c M u n n remark, has only been recorded in association with the Cyclopean condition. It appears to me that this m a y possibly imply that the reversion is so general that it has affected the normal metamorphosis, that there is in fact a partial reversion to symmetry of the head (as well as to symmetry of the pigment and scales) exemplified by such a want of harmony in the migrations of the eye and dorsal fin, as I have already suggested may be the mechanical cause of the Cyclopean result. It is evident, however, from the usually normal colour condition of specimens in which the structural abnormality of the head is confined to the projection of a small part of the dorsal, that such abnormality is not necessarily accompanied by any tendency towards a reversion of pigmentation, nor does it appear to be indicative of even the slightest reversion towards symmetry of the head. In the Sole now under consideration, as w e have seen, the under surface is white. The muscles of the blind side are reduced in the normal manner, and the ciliation of the scales is feeble. The dorsal fin has completed its usual forward migration, but the eyes remain practically symmetrical. W e have seen that there is no reason to suppose that ambicolorate examples, whether Cyclopean or structurally normal, differ in any of their habits from perfectly normal fish, since Cunningham's observations of the behaviour of a living Cyclopean |