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. TEETH. 
Dorsal: about 60 rays. Teeth on the jaws, of moderate size, 
Pectoral: vo rays. more developed on the blind side 
Ventral: 6 rays. than on the coloured one. No 
Anal: 39 to 45 rays. teeth on palate or vomer. 
Caudal: 14 rays. 
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, 
