CLASSIFICATION AND ADAPTATION 25 



on the upper and lower sides, and there is almost 

 complete symmetry in these parts of the skull. 

 In Soles and Plaice, on the other hand, whose food 

 consists of worms, molluscs, etc., living on or in the 

 ground, the jaws of the lower side are well developed 

 and strong, these of the upper side diminished, 

 and teeth are confined to the lower side. Here 

 it is not a question of the jaws being twisted, but 

 simply unequally developed. There is no general 

 and constitutional asymmetry of head or body, 

 but a modification of different organs independently 

 of each other in relation to external conditions — 

 light, food, movement. 



On the other hand, let us consider some of the 

 diagnostic characters by which species and genera 

 are distinguished in the Flat-fishes or Pleuro- 

 nectidae. The genus Pleiironectes is distinguished 

 by the following characters : eyes on the right side, 

 mouth terminal and rather small, teeth most de- 

 veloped on the blind (left) side. Of this genus there 

 are five British species, namely : — 



P. platessa, the Plaice : scales small, mostly 

 without spinules, reduced and not imbricated, 

 imbedded in the skin ; bony knobs on the head 

 behind the eyes, red spots on the upper side. 



P. flesus, the Flounder : no ordinary scales ; 

 rough tubercles along the bases of the marginal 

 fins and along the lateral line ; these are modified 

 and enlarged scales; elsewhere scales of any kind 

 are absent. 



In these two species the lateral line is nearly 

 straight, having only a slight curve above the 

 pectoral fin. 



P, limanda, the Dab : scales uniform all over 



