THE FLOUNDER 253 



In most Pleuronectids the fins are composed of 

 articulated rays only ; the dorsal and anal are long 

 and fringe the dorsal and ventral edges of the flat 

 body ; the caudal has the normal homocercal 

 structure and the pelvics, formed of six or fewer 

 rays, are placed in advance of the pectorals. In 

 the adult fish the air-bladder is absent ; as in the 

 Percoid fishes the praemaxillaries form the entire 

 upper border of the mouth, and another Perch-like 

 feature is the firm and direct attachment of the 

 pelvic bones to the clavicles. 



The most generalized of all the Flat-fishes is the 

 one named Psettodes erumei, found on the coasts of 

 Africa, India, and China. In this species sinistral 

 and dextral individuals are equally numerous, and 

 it differs from all other Pleuronectids in that the 

 eye of the blind side comes on to the top of the 

 head only, whilst the dorsal fin does not extend 

 forward on to the head, and anteriorly is formed of 

 slender spines ; moreover, each pelvic fin is formed 

 of a spine and five soft rays, as in a Perch. The 

 wide mouth and strong sharp teeth give the clue 

 to the retention of these Percoid features; this is 

 evidently an active and predaceous fish, often 

 swimming by strokes of the tail. It is in the Flat- 

 fishes which usually progress quietly by undulating 

 movements of their fringing fins that the spines 

 have become reconverted into articulated rays and 

 the dorsal fin has grown forwards on to the head, 

 and the more they have become accustomed to 

 living and feeding on the bottom the more has 

 their asymmetry affected not only the eyes and 

 skull, but the mouth and dentition, the paired fins, 

 and the position of the vent. 

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