THE FLOUNDER 95 



pectoral and ventral fins ; they then turn over on one side (in 

 most flat-fishes it is the right), and for the rest of their lives 

 continue to rest and move upon that side only. The exposed 

 side takes on characteristic colouring, usually in close assimila- 

 tion to the surrounding ground ; while the side next the 

 ground becomes white, like the bellies of other fish, and is 

 called the " blind " side. 



Of course this change in position causes one eye to rest 

 upon the ground, to the manifest detriment of its functions ; 

 but this is rectified by its withdrawal from the blind side, and 

 its rearrangement on the coloured side. Naturalists are not of 

 one opinion how this is effected. Some consider that the eye, 

 yielding to its natural tendency to turn towards the light, 

 turns round upon its axis, and, forcing its way through the 

 cartilages and bones of the head, assumes a position beside and 

 in front of the other eye upon the upper side. Others hold 

 that it moves round from below, carrying with it the whole 

 fore-part of the head. Whatever be the exact process, it is a 

 very singular one, and highly suggestive in regard to the 

 forces at work in the modification of form in living organisms. 



Flat-fishes seem to be a very modern type, for the only 

 example found in a fossil state is a species of turbot from the 

 tertiary beds of Mount Bolca. 



The Flounder (Pleuronectes flesus] 



FIN FORMULA. 

 Dorsal : about 60 rays. 

 Pectoral : 10 rays. 

 Ventral : 6 rays. 

 Anal: 39 to 45 rays. 

 Caudal : 14 rays. 



TEETH. 



Teeth on the jaws, of moderate size, 

 more developed on the blind side 

 than on the coloured one. No 

 teeth on palate or vomer. 



In Britain the fresh-water representatives of the flat-fishes 

 are of modest proportion, never approaching in size to the 

 lordly halibut, whereof the weight is sometimes reckoned, 



