148 PLEURONECTID^. 



course, the pectoral fin for the most part is brought into 

 action for this purpose. 



In separating this numerous family into subordinate groups, 

 it is convenient and usual to divide them principally accord- 

 ing as when the belly is towards the observer the head and 

 eyes are directed to the right or left. But this particular 

 structure is not known to exert any influence on the habits 

 of the individual species; and, therefore, in combination with 

 this it is of importance to include in our consideration other 

 material variations of form; which, therefore, are placed among 

 the essentials of the definitions of the genera. 



