582 MALACOPTEKYGII. 



tinct. One of the dorsal fins is fronted with a strong bony 

 spine. The bones or rays of this fin are so contrived as to act 

 in concert for suddenly elevating it at the pleasure of the 

 fish. It is a singular fact that if the foremost or largest ray be 

 pressed ever so hard, it will not stir ; and yet if the last or least 

 ray be pressed very slightly, the other two immediately fall 

 down with it, just as a cross-bow is let off by pulling down the 

 trigger. To this peculiarity there is a reference in the name 

 of the typical genus Balistes; The fishes of this genus are cov- 

 ered with large and hard rhomboidal scales. The DUSKY BAL- 

 ISTES, B.fuliginosus, (Lat. dusky,) twelve inches long, is found 

 off the coast of the United States. The MASSACHUSETTS FILE- 

 FISH, Monocanthus, (Gr. a single spine,) has very small scales, and 

 a single large spine in place of the first dorsal fin. Its length is 

 from three to five inches. The LONG-TAILED UNICORN-FISH, 

 Aluteres cuspicquda, (Lat. pointed or spear-tailed,) has the skin 

 covered with small and almost invisible granules. Length from 

 six to nine inches. 



(30) Oslracionida, (Gr. ostrakion, a shell' or covering.) 

 TRUNK FISHES. 



These are a group of singular fishes, found principally in the 

 American and Indian seas. They are enveloped in a bony 

 crest or covering, so united as to form an inflexible shield, leav- 

 ing only the tail, fins, mouth, and a small part of the gill-open- 

 ings capable of motion, passing through openings in the Arma- 

 dillo-like shield. There are no ventral fins, and but one dorsal. 

 These fish have little flesh, but a large liver, abounding in oil. 

 The surface, in some species, is armed with spines. There is but 

 one North American genus, Lactophrys. The species L. cameli- 

 nus, (camel-like,) has the back elevated into a spine, and is three 

 and a half inches in length. 



GANOIDS. 



These are characterised by having the scales bony, and 

 covered, externally, with enamel, generally angular and contin- 

 uous. Most of them are extinct species. 



(31) Saurida. This name has been employed by Agassiz to 

 designate the fishes of this group, which also comprises the 

 Polypterus, (Gr. polus, much or many ; pteron, fin,) of the Nile. 

 This latter (fresh-water) fish is usually about eighteen inches in 

 length, and partakes both of the osseous and cartilaginous kinds; 

 but is thought by some to be " most nearly allied to those species 

 of the genus Esox, which are furnished with large, long, and 

 bony scales." Its color is sea-green. It is called by the Egyp- 



