In form and general appearance the Spine-tails resemble the Chætodons, with which Linnæus associated them. They are marked by the body being short, and greatly compressed; it has been stated, that "the depth of the body, measured from the dorsal to the pectoral fins, is always equal to, and often exceeds, its length from head to tail;" but this is certainly not the case with the most typical forms. There are teeth only in the jaws; these are trenchant and denticulated, like the teeth of a very fine comb: the mouth is small, advanced, but not projectile. The fins are much developed; they are supported by numerous slender rays, and are destitute of scales; the dorsal and anal are long; and the caudal is forked, the points frequently being produced into filaments. But the most remarkable character of these fishes is the presence of moveable spines set on each side of the fleshy part of the tail, the points and edges of which are as sharp as those of lancets. With these weapons they inflict dangerous wounds on the hands of such as handle them incautiously. These lancets have procured for them the familiar name of "Doctors," by which they are generally known to sailors and colonists.
Genus Acanthurus, (Lacep.)
The sides of the tail in this genus are armed each with a single lancet only; the body is oval, covered with minute scales; the mouth very small and projecting; the dorsal is long and undivided, supported, as is also the anal, by numerous rays, of great slenderness, set very
