362 Prof. W. Thomson on Steenstrup's views 



upon one side as upon a ventral surface, swimming by the un- 

 dulation of the horizontal unpaired fins. 



A new system of equilibrium is established for the Flounders, 

 in which the dorsal and ventral instead of the lateral halves 

 become symmetrical in outline and are equipoised. In most of 

 the Pleuronectidse, e. g. in Platessa, Hippoglossus, and Solea, the 

 left side is the blind side ; but in some groups, e. g. Rhombus 

 and the young forms termed provisionally Plagusia, the right 

 side is blind, and the left side bears the eyes. In both of these 

 groups, however, there occasionally occur " wrong Flounders " 

 — dextral Flounders in the sinistral group, and vice versa. 



Besides "right'' and "wrong'' Flounders, we have in each 

 group " double Flounders," individuals in which both sides are 

 nearly equally developed and coloured. These have the eyes 

 placed, one in its ordinary position on the eye- side, and one on 

 the top of the head. They approach the ordinary fish-form, 

 and swim vertically; nevertheless they must be regarded as 

 monsters among the Pleuronectidse. 



The external obliquity of the Flounders is accompanied by 

 important structural deviations. The muscular system of the 

 blind side is much more feebly developed than that of the eye 

 side. The fins are smaller, and, even in the unpaired fins, the 

 lower halves of the fin-rays are weaker. The gills and gill- 

 covers are smaller, and the skin is usually less fully developed. 

 The anterior portion of the face is twisted round, so that the 

 whole mouth is bent towards the blind side, while at the same 

 time the posterior part of the face, in which the eyes are placed, 

 is strongly pressed over to the eye side. There is added to this 

 a remarkable removal of certain portions of the most central part 

 of the face ; and this it is which, in connexion with the twisting 

 just mentioned, ultimately brings the eyes into their normal 

 position for each species — both on the same side of the head. 

 This latter condition is universal among Flounders, and may be 

 regarded as essentially characteristic of this curious group. 



II. 



To understand this clearly, we must determine accurately the 

 position of the eyes of a Flounder with reference to the surround- 

 ing parts. The two eyes are not placed in a straight line one 

 above the other : the upper eye is either somewhat behind or 

 somewhat before the lower — usually behind; before in the Soles 

 and the Plagusise. The eyes are in opposite positions ; their 

 upper margins are turned towards one another, and the lower 

 margin of the upper eye is turned upwards towards the dorsal 

 line of the fish. Between the eyes there stretches a firm bony 

 partition (Mellembalk) formed of definite cranial bones. In the 



