196 FISH GALLERY. 



beak is divided by a median suture, whereas the lower is un- 

 divided, whence the name Triodont or three-toothed fishes. 

 There is no spinous dorsal fin, and there are no pelvic fins. The 

 abdomen has a dilatable sac. The body is covered with small 

 spiny laminae of bone. There is a single species, Trio don 

 bursarius, 903, of the Indian Ocean. 



In the File-fishes (family Balistidae) the teeth remain separate 

 and have the form of incisors. There is a spinous dorsal fin; the 

 pelvic fins are absent, or they take the form of a single short spine 



Fig. 93. — File-fish, Balistes vidua. 



at the end of the long pelvis (see skeleton 912). The principal 

 genera are Balistes (File-fishes or Trigger-fishes, fig. 93), Mona- 

 canthus and Aluteres. They are all inhabitants of tropical and 

 temperate seas. The largest species of the genus Balistes grow to 

 three feet. The body is protected by closely-fitting, hard, 

 rhomboidal scales. The teeth are powerful and enable the animal 

 to break off pieces of coral, etc., on which it feeds, and to bore into 

 or chip the edges of shells of molluscs ; the Pearl Oyster is 

 particularly liable to attack (see the shells 920). One species 

 of Balistes (B. capriscus, 911) is occasionally caught in British 

 seas ; the exhibited specimen was obtained on the Cornish coast. 

 Eleven other species of Balistes are exhibited (907-910 and 913- 

 919) which serve to show the wide range of colouring and marking 

 observable in the genus. Erythrodon (921) is a subgenus of 

 Balistes distinguished by the possession of red teeth. Monacanthus 



