INTRODUCTION. 85 



acting power -svliich regulates the motions of the 

 fins, which are onh' accessaries. In all the long 

 snake-formed species it acts an important part ; 

 and although, in the eels and lampreys, the power 

 of the posterior extremity in the water is increased 

 by being bordered by a narrow fin, yet the motions 

 of these fish, when they have to travel over portions 

 of land, or any dense weeds, which is very fre- 

 quently the case, are conducted entirely by the 

 spine, the finny border being there quite unneces- 

 sary. Our first Plate, illustrating a curiously 

 marked species, will illustrate this ; — it is 



