432 THE MECHANICAL FUNCTIONS. 



the particular situations and circumstances in 

 which they are placed. The dorsal fins, which 

 are more especially useful for steadying the 

 body, are longest in those fishes which inhabit 

 the most stormy seas. The most voracious 

 tribes, which incessantly pursue their prey, are 

 furnished with most powerful muscles, and 

 possess the greatest means of rapid progression. 

 On the other hand, many of the more pacific, 

 and weaker species are studiously guarded 

 by a dense and hard integument, serving as a 

 shield against the attacks of enemies, and often 

 armed with sharp points, which are sufficient to 

 repel the most daring assailant. The Batistes 

 is covered with scales of singular hardness 

 closely set together, and frequently having 

 rough edges. The Ostracion, or trunk fish, 

 instead of these scales, is provided with a kind 

 of coat of mail, composed of osseous plates, 

 curiously joined together, like a tesselated pave- 

 ment, and reminding us of the arrangements 

 we have seen adopted in the calcareous cover- 

 ings of the echinida. 



Some of the cartilaginous fishes are in like 

 manner protected by calcareous plates, appended 

 to the integuments. There is a row of plates of 

 this kind, of a quadrangular shape, which pass 

 along the middle of the back in the sturgeon : 

 and the whole body of the Ostracion, or Trunk- 

 fish, is covered with osseous scales. All these 



