366 Prof. W. Thomson on Steenstrup's views 



its eye in and up through the head, out to the other side, and at 

 last squints itself into a perfect Flounder/^ 



The other specimens in the museum correspond with the 

 stage fig. 2 A, but are not so far advanced. They have charac- 

 ters which indicate that they belong to several species and even 

 genera. It seems, in fact, that at least a whole group of Pleuro- 

 nectidse pass through similar early stages ; and, from the struc- 

 ture of the mature skulls, it is more than likely that this method 

 of the production of the obliquity of the eyes is universal in the 

 family. 



IV. 



The author analyses two direct observations which have usu- 

 ally been supposed to support the view that the eyes acquire 

 their final position, both on one side of the head, by a simple 

 torsion of the anterior portion of the head in the young fish. 

 The first of these was made by Professor Van Beneden, and was 

 published by him in the 'Bulletin de PAcademie Royale de 

 Belgique,' t. xx. 1853 ("Note sur la Symetrie des Poissons 

 Pleuronectes dans leur jeune age^'). 



An extremely minute fish, apparently only recently extruded 

 from the egg, was taken in a fine-meshed net along with 

 Shrimps. The eyes were unsymmetrical — one in its ordinary 

 position, the other higher up, on the top of the head ; the dorsal 

 fin came down on the back of the head, but not to the eye ; and 

 Van Beneden concludes that a further twist would have brought 

 the eye further down, and that the dorsal fin would then have 

 extended past it over the head. 



Prof. Steenstrup gives good reasons for doubting that this 

 very young form was a Flounder at all, and is rather inclined 

 to refer it to Gunellus, or some other of the Blenny group. At 

 all events, admitting that it was a Pleuronectid, there is nothing 

 in its structure by any means conclusive against the eye having 

 been ready to perform its migration according to Prof. Steen- 

 strup's view, at a later stage. 



The second observation is by A. Malm, Curator of the Gote- 

 borg Museum, published. shortly after, and independently of, 

 Van Beneden^s paper (" Ofversigter Kgl. Sv. Vetenskaps Acade- 

 mien, 1854^'). A young Rhombus barbatus (Clocq.), 20 millim. 

 long, was found swimming obliquely near the surface of the 

 water. Its colour was nearly the same on both sides ; the lower 

 eye was in its usual place, but the upper eye was on the top of 

 the head. The dorsal fin ceased immediately behind the eye. 



Malm assumes, 1st, that he had before him an ordinary 

 stage in the development of the species towards its normal form; 

 2ndly, that the right eye had reached its position at the top of 



