AN AMBICOLORED TURBOT 149 



Filhol, 1 and of which an example is shown in a stuffed Turbot 

 in the Royal Scottish Museum, the anterior attachment of the 

 dorsal fin ends behind the eye, while the eye itself lies on the 

 top of the head, overhung by the free projection of the fin. In 

 such cases it has been argued by those and other writers that 

 the fin has become detached owing to the detention of the eye 

 on the top of the head ; and such an explanation would seem 

 to apply to the present case, in spite of the doubt cast by Prof. 

 Bateson upon its validity in the case described by Duhamel. 



That the movement of the right eye from the under to the 

 upper surface was in some measure delayed, is indicated by the 

 fact that, in a well-grown fish, it has not reached its normal 

 proximity to the left eye. It is reasonable to assume either 

 ( i ) that the eye was temporarily arrested on the top of the head 

 (it has been arrested there so frequently in other cases), and 

 that thus it became impossible for the fin to continue its 

 growth and still to retain its attachment to the head ; or (and 

 I suggest this as the more probable explanation, since it avoids 

 postulating an arrestment followed by a resumed migration) 

 we may assume (2) that the eye commenced its migration 

 at a period later than usual in the development of the fish, 

 that the fin had already extended so far forward that it 

 abutted against the eye migrating across the top of the head, 

 and that, continuing to grow forward, it was compelled to 

 separate from the head. If we suppose that the fin had 

 extended still further forward, the migrating eye may even 

 have been forced to pass underneath its anterior portion, 

 which, thus detached, would be unlikely at a subsequent 

 stage to regain its attachment. In either case the eye, once 

 over the ridge, continued its migration, and the attached 

 portion of the fin, freed from obstruction, was pushed forwards 

 by the increase of the elements of which it was composed. 



Another possible explanation which, however, fails to 

 account for the abnormal distance between the eyes is 

 that the separation of the fin is an isolated variation, a 

 phenomenon comparable to the detachment, occasionally 

 found in the Skate,' 2 of the anterior portions of fins. 



1 Filhol, "Description d'un cas de monstruosite obsarve sur un Rhombus 

 vulgaris (Cuv.)," Bull. Soc. Phil., p. 55. Paris, 1890. 



2 See Traquair, R. H., " Note on an abnormally developed Thornback," 

 Ann. Scot. Nat. Hist., i., 1892, p. 29. 



