44 ORDER TELEOSTEI; CLASSIFICATION. 



skin is usually covered with horny scales, but sometimes quite 

 naked, and in a few instances more or less protected by bony 

 plates. The gills are quite free, and protected by a movable 

 operculum, which is often rather complicated in its structure ; 

 the bulb of the aorta is thickened, but not muscular, and destitute 

 of the internal valves of the Selachian and Ganoid Fishes ; and the 

 intestine exhibits no trace of the spiral valve. 



602. This order contains the great majority of the existing 

 species of the class of Fishes, and its study is proportionately 

 difficult, both from the number of groups into which it is neces- 

 sary to subdivide it, and the difficulty which often occurs, as in 

 all large groups of animals, in finding good characters for its 

 subdivision, the different families often melting into each other 

 so insensibly that it is hard to say where one ends and the other 

 begins. 



603. The great variety of organisation presented by the 

 Bony Fishes has induced their division into six primary groups, 

 or sub-orders, each including several families. These are, 



A. PLECTOGNATHI, in which the bones of the upper jaw and 

 face are firmly attached to, or amalgamated with, those of the 

 skull. 



B. LOPHOBRANCHII, with the branchial filaments arranged in 

 little tufts along the branchial arches, and the face produced into 

 a long snout. In both these groups the opercula are confined by 

 the skin, so that only a small aperture is left for the escape of 

 the water. In the remaining groups, the bones of the upper jaw 

 and face are more or less moveable, the gills are not tufted, and 

 the opercular aperture is usually large. 



C. ACANTHOPTERI, with spinous rays in the anterior part of 

 the dorsal fin, or in the first dorsal when there are more than 

 one, and the inferior pharyngeal bones ( 541) distinct. The 

 air-bladder, when present, is completely closed. 



D. PHARYNGOGNATHI, in which the inferior pharyngeal bones 

 are completely amalgamated, and usually armed with teeth. The 

 air-bladder is closed. 



E. ANACANTHINI, with no spinous rays in the fins, and a 

 completely closed air-bladder. 



