FISHES. 327 



upper surface, and the caudal fin, though heterocercal in 

 the young, is homocercal in the adult.* The skull is cov- 

 ered with numerous bony plates, and the body is covered 

 with either cycloid or ctenoid scales. Sometimes (trout) 

 scales are apparently lacking, but this apparent absence 

 may be due to their small size and their being buried in the 

 skin. The gills are covered by an operculum. Of the 

 internal features which characterize the group may be 

 mentioned the absence of a spiral valve in the intestine, 

 the presence of an arterial bulb in the heart, and, very 

 frequently, of a swim-bladder. 



The ten thousand species of bony fishes are variously 

 subdivided by naturalists accordingly as different structures 

 are made the basis of classification. One of the simplest 

 of these schemes recognizes six subdivisions or orders, 

 and is adopted here. To which does the specimen you 

 studied belong? 



ORDER I. PHYSOSTOMI. 



Bony fishes in which the gill-filaments are arranged on 

 the branchial arches like the teeth of a comb; with the 

 premaxillary and maxillary bones movable (p. 24); the 

 dorsal, anal, and ventral fins supported only by soft rays 

 (p. 23) ; the ventral fins, when present, placed near the 

 vent. An air-bladder is almost always present and 

 retains its connection with the throat throughout life 

 (p. 310). The scales are usually cycloid (p. 318). Most 

 of the species belong in fresh water. 



The catfishes and horned pout, with long filaments or 

 barbels about the mouth, belong here. In our eastern 

 waters the species are small, but in the Mississippi basin 



* In a very few the tail remains diphycercal throughout life. 



