Page:Natural History, Fishes.djvu/303

Rh covered with minute rough points, except in regular lines which cross each other, and mark off the whole surface into lozenge-shaped divisions. 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 commonly 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 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 effected 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. "The 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