440 MK. B. 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 character- 

 istic of the hypothetical Pleuronectid ancestor as symmetry of 

 colour, the presence of spines or tubercles on both sides of 

 ambicolorate flat-fish seems to me 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 charac- 

 teristic 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 tubei'cles, 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 I'eversiou would inculcate 

 that the reversion to colour-acti\ity and symmetry of scales is 

 confined to certain areas of the derma. Total ambicoloration, as 

 Messrs. Cunningham and MacMunn remark, has only been recorded 

 in association with the Cyclopean condition. It appears to me 

 that this may 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) exemphfied 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 ui 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 we 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. 



We have seen that there is no reason to suppose that ambi- 

 colorate 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 



