THE TRUNK-FISHES 



Family LXIV. Osiraciidce 



These fishes may at once be known by the short, cuboid, 

 triquetous or pentagonal body, covered by a carapace formed of 

 firmly united polygonal bony patches, the jaws, bases of the 

 fins, and the caudal peduncle being the only parts free and 

 covered with smooth skin. 



The locomotion of the trunk-fishes is very peculiar. The pro- 

 pelling force is exerted by the dorsal and anal fins, which have 

 a half-rotary, half-sculling motion, resembling that of a screw 

 propeller; the caudal fin acts as a rudder, save when it is needed 

 ■for unusually rapid swimming, when it is used as in other fishes. 

 The chief function of the broad pectorals seems to be that of 

 forming a current of water through the gills, thus aiding in 

 respiration, which would otherwise be difficult on account of the 

 narrowness and inflexibihty of the branchial apertures. When 

 taken from the water one of these fishes will live for 2 or j 

 hours, all the time solemnly fanning its gills, and when restored 

 to its native element seems none the worse for its experience, 

 except that, on account of the absorbed air, it cannot at once 

 sink to the bottom (Goode). 



The family contains one genus, Ostradon, with 4 American 

 species which may be distinguished as follows: 



a. Carapace without spines anywhere; triqueter, 490 



aa. Carapace with distinct spines, at least on the ventral rijges 



behind. 



b. Frontal spines none. 



c. Carapace closed behind the dorsal fin; body everywhere with 



round dark spots ; bicaiidalis, 490 



cc. Carapace open behind the dorsal fin; body mottled with 



paler; fn'gonus 490 



bb. Frontal region with 2 strong spines like horns;- -iricornis, 490 



These fish are all sluggish in their habits, living in shallow 

 water at the bottom about reefs and feeding upon minute animal 



489 



