Balistidae and Monacanthidae - triggerfishes and filefishes
Triggerfishes and filefishes are sometimes listed in the same family, Balistidae, under the common name "leatherjackets". Both families have rough thick skin made of scale plates. They have rudimentary or no pelvic fins, usually just a bony protruding knob. Mouths are small but with well-developed teeth. Gill openings are short vertical or oblique slits in front of pectoral fin.
Balistidae, the triggerfishes, have three dorsal spines. The first spine locks in an upright position. The second smaller dorsal spine acts as a trigger to depress the first spine. Scales above the pectoral fin may be enlarged forming what is called a tympanum. Its dorsal and pectoral rays are branched at the ends.
Monacanthidae, the filefishes, have two dorsal spines. The first spine is long and usually barbed. The second spine is minute. No tympanum is present. Their fin rays are not branched at the ends.