FILE-FISHES. 289 



covered with minute rough points, except in 

 regular lines which cross each other, and mark 

 off the whole surface into lozenge-shaped divi- 

 sions. The appearance is so much like that of 

 a smith's rasp or file, as to have suggested the 

 name of File-fish, by which the genus is com- 

 monly known. There are two dorsals, of which 

 the first is composed of three spinous rays. The 

 ventrals have each a strong bony spine, without 

 membranes ; the other fins are generally much 

 developed. In some species the sides of the tail 

 are armed with rows of sharp prickles. 



The first ray of the front dorsal is a strong 

 pointed bone, studded along its anterior side vrith 

 hard rough points. It is a powerful weapon, the 

 effect of which is heightened by an interesting 

 contrivance. When it is erected, which is effect- 

 ed with a kind of spring or catch, no force can 

 depress it, but the least touch applied to the 

 second or third, lowers the whole fin in a moment. 

 Hence it is named Balistes, from the ancient 

 machine which shot darts at an enemy. The 

 structure by which this effect is produced is thus 

 explained by Professor Owen. *' Tbe first spine 

 is articulated by a very remarkable joint to [a 

 broad bony plate on the top of the head] ; its 

 base is expanded and perforated, and a bony 

 bolt passes freely through the ring. When this 

 spine is raised, a depression at the back part of 

 its base receives a corresponding projection from 

 the contiguous base of the second^ ray, which 

 fixes it like the hammer of the gun-lock at full 

 cock ; and it cannot be forced down till the small 

 spine has been depressed, as by pulling the 

 trigger : it is then received into a groove on the 



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