Fig. 193.—Acanthurus leucosternum, Indian Ocean.
At an early period of their growth these fishes present so different an aspect that they were considered a distinct genus, Acronurus. The form of the body is more circular and exceedingly compressed. No scales are developed, but the skin forms numerous oblique parallel folds. The gill-cover and the breast are shining silvery.
Naseus.—Tail with two (rarely one or three) bony keeled plates on each side (in the adult). Head sometimes with a bony horn or crest-like prominence directed forwards. Ventral fins composed of one spine and three rays. From four to six spines in the dorsal; two anal spines. Scales minute, rough, forming a sort of fine shagreen. Air-bladder forked behind. Intestinal tract with many circumvolutions.
Twelve species are known from the tropical Indo-Pacific, but none of them extend eastwards beyond the Sandwich Islands. In their mode of life these fishes resemble the Acanthuri. Likewise, the young have a very different appearance, and are unarmed, and were described as a distinct genus, Keris. One of the most common species is N. unicornis, which, when adult (22 inches long), has a horn about 2 inches long, whilst it is merely a projection in front of the eye in individuals of 7 inches in length.
Prionurus is an allied genus with a series of several keeled bony laminæ on each side of the tail.
Fig. 194.—Naseus unicornis.
Second Family—Carangidæ.
Body more or less compressed, oblong or elevated, covered with small scales or naked; eye, lateral. Teeth, if present, conical. No bony stay for the præoperculum. The spinous dorsal is less developed than the soft or than the anal, either continuous with, or separated from, the soft portion; sometimes rudimentary. Ventrals thoracic, sometimes rudimentary or entirely absent. No prominent papilla near the vent. Gill-opening wide. Ten abdominal and fourteen caudal vertebræ.