28 



basis for their division ; one division has the scale smooth and 

 rounded, the other deeply toothed at the edges. 



Perhaps the most singular Fishes in the aquarium are the 

 Brills, Plaice, Flounders, and Soles. These Fish are not depressed 

 as the Skate, which swims with its back uppermost ; but they are 

 compressed, and swim with one side upwards, the upper side 

 being generally dark coloured and the under nearly white. 



In the Holibut, Plaice, and Sole, the eyes are directed towards 

 the right ; in the Turbot and Brill, towards the left ; in the 

 Flounder, to the left, but nearly as often to the right. Several of 

 the species commonly have more or less of the bro^vn colour on 

 the under side, and others more or less wanting in colour on the 

 upper side. 



It is very difficult to imagine what can be the determining 

 cause as to which side a species should incline to, and still more 

 in the case of individuals of the same species. 



I believe myself the prevailing current of the sea frequented, 

 combined Avith the direction of the shore, gave an advantage to 

 one side over the other, and then by heredity the dextral or 

 sinistral advantage became fixed. 



Flounders, inhabiting the mouths of rivers and seeking their 

 prey sometimes on the left and sometimes on the right bank, have 

 not yet had their eyes determinately fixed either sinistral or 

 dextral. 



But let this be as it may, one fact is certain, that these asjTn- 

 metrical flat fish are in their young state symmetrical, or, in 

 other words, have their eyes like other fish, one on the right and 

 the other on the left side of the body ; and that afterwards the 

 head is bent either to the left or right and the eye which would 

 then have been underneath is moved round to the upper side. 



An adult Turbot has been figured by Scleep, in which the eyes 

 were still one on each side of the head. Other very curious cases 

 have been figured both by Yarrell and Couch, in which the 

 twisting round of the head had been imperfectly effected. All 

 such abnormal fishes are equally or similarly coloured on both 

 sides ; the fish probably swam with either of its sides uppermost, 

 and Dr. Traquaire, in his paper on the subject published in the 

 the Transactions of the Linnean Society, observes that the " bony 

 tubercles usually characteristic only of the ocular side of the 

 Turbot are found equally distributed on the eyeless side in these 

 abnormal specimens." 



