MINOR ABNORMALITIES— AMBICOLORATION 57 



(a) with shortening of the cranial attachment of the dorsal fin, so that the tip of this fin 

 reaches forward for some distance as a free hook-shaped process; 



(b) with the presence of spines, tubercles, or rough scales on the under side simulating in some 

 degree those which are normally found on the upper side. In addition, there may be unusual 

 thickness of the muscular layers, and undue pigmentation of the peritoneal lining, on the under side ; 



(c) often also with abnormality in the position of the eye belonging to the under side, which 

 looks as if it had not completed its migration. In well-marked instances the eye in question may 

 not have travelled further round than the (apparent) dorsal edge of the head and may accordingly 

 be partly visible from the blind side of the fish, as in the so-called "cyclopean" condition. Or more 

 commonly, both eyes may be on upper side of the head, but lying a little wider apart than normal 

 and approximately in the same transverse line. 



Eeferences to the earlier literature on the subject of these abnormalities are given by Bateson 

 (19, 466-473), and Cunningham and MacMunn (50a). The former quotes Steenstrup as stating 

 that hooking of the dorsal fin has been observed in all flat-fish, except the halibut (Hippoglossus). 

 For the turbot (Rhombus maximus) Cunningham makes the generalisation that hooking of the dorsal 

 fin is present in all ambicoloured specimens which show pigmentation on the under side as far 

 forward as the preopercular region, the tip of the dorsal fin, and the lower jaw. In this species 

 the rest of the under side of the head seems hardly ever to be pigmented. In ambicoloured 

 flounders, however, the whole of the under side may show pronounced pigmentation. 



Of instances later than those which are noticed by Bateson (loc. cit.), we may quote the 

 following : 



(1) Cases in which the ambicoloured condition is associated both with hooking of the dorsal fin 

 and with abnormality in the position of the eye : 



Plewonectes fiesus, Cunningham (50 a), Elmhirst (G4-), M'Intosh (155, two specimens) ; 



Pleuronectes pkttessa, Cunningham (50 a, noted by Dr. Brandt, Kiel) ; 



Pleuronectes italicus, 1 Ninui (169); 



Pleuronectes limanda, Elmhirst (64-); Vaillant (261); 



Rhombus maximus, Cunningham (50 a), Ritchie (206, eyes approximately normal in 



position); Elmhirst (64.), Sacchi (211), M'Intosh (155, four 



specimens) ; 

 Rhombus laevis, Filhol (69), Johnstone (119); 



Solea vulgaris, Ninni (169), Cuenot (46). 



(2) Cases in which the ambicoloured condition is associated with a fin abnormality, but 

 apparently not with any marked abnormality in the position of the eye : Rhombus maximus, 

 Rh. laevis, Plewonectes fiesus, PI. platessa Cunningham (50a, 50), PI. platessa, Elmhirst (64). 



The Rh. laevis mentioned under (2) above as having been described by Cunningham is of 

 particular interest. In the first place it is a reversed specimen, and in the second place the upper 

 surface of the body is extremely deficient in pigment, except anteriorly, where a certain amount occurs 

 in the head region, especially between the eyes and around the dorsal eye. The lower side, on the 

 other hand, is deeply, but not uniformly, coloured, pigmentation being absent from the head and 

 from the dorsal region above the head. The specimen was very young (4 '4 cm. in length). 



In this connection it may be noted that one of the ambicoloured plaice recorded by Elmhirst 

 (64) was almost white on the upper side in the posterior quarter of the body, and had the dorsal fin 

 not reaching to the head but ending over the origin of the pectorals. 



Mode of Origin and Causation. It seems to be certain that in most flat-fishes (Plagusia being 



1 A careful analysis of the differences between this specimen and a normal PL italicus is given by Ninui (1<J0), from which 

 the following points may be noted, in addition to the three principal ones which have been named : The form of the body is 

 elongated instead of being oval ; the mouth opening is almost vertical instead of being oblique ; there are well-marked 

 tubercles on the under side ; the interorbital space measures 4 mm. instead of 1 mm. The lateral line makes a very slight 

 bend above the pectorals instead of a somewhat pronounced curve. The contour of the preopercular is well marked instead of 

 being hardly distinguishable. The pelvic fins are equal, and the pectoral fin on the upper side is as large as that on the blind 

 side, instead of being somewhat smaller. 

 H 



