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 Jj^nown. 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 with 
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 
u 
