rows counted diagonally from the lateral line to the base of the dorsal fin, 

 and ten rows from the lateral line down to the base of the anal fin. 



Head 



The position of the mouth, terminal, superior or inferior, is important. 

 The bones which normally form the margin of the upper jaw are the pre- 

 maxillary and the maxillary. These should be carefully located. In some fishes 

 the prcmaxillary is long and underlies the maxillary, forming the entire mar' 

 gin of the jaw, while in other fishes both bones reach the margin. In some 

 fishes both bones are hinged at the front, free at the back, and, when swung 

 forward, support skin folds to form a tube with which food can be sucked up. 



The opercular bones should be located and their names learned. Varia' 

 tions in their shapes and even in their number are found. In the catfish family, 

 Ameiuridae, for example, the subopercular is not developed. 



Gill Membranes 



These are the folds of skin which form the floor of the gill chamber, and 

 which are usually supported by cartilaginous or bony bars called the branchio' 

 stegal rays. The membrane from one side may extend almost directly across 

 to, and fuse with, the corresponding one on the other side, or both may extend 

 far forward before joining. The fused membranes may or may not be joined to 

 the isthmus or section of the body directly beneath them. 



Teeth 



Teeth may be found not only on the jaws but also on the roof of the 

 mouth and on the tongue. Many fishes have teeth on the pharyngeal bones 

 behind the last gill arch, even though they may not have any in the mouth. 

 These pharyngeal teeth are often of great importance in taxonomic work, es- 

 pecially in the minnow family, Cypnnidae. The pharyngeal bones may be 

 removed by reaching in under the operculum with small forceps. The bones 

 lie directly behind the gills and correspond to a fifth gill arch. If they are 

 carefully removed, cleaned and dried, the teeth may be examined under a 

 lens. The bones from both sides should be examined, as they are not always 

 alike. The lower pharyngeals are the most highly specialized and most com- 

 monly used in identification. The tooth formula is usually given for both 

 members of a pair: thus 4 - 4 means one row of four teeth on the right and 

 left pharyngeals, 2-4-4-1 means that there are two rows on each, the left 

 with two in the outer and four in the inner row, the right with four in the 

 main or inner row and one in the outer row. The nature of the surface of 

 the pharyngeal teeth is also important and gives a clue to the food habits. 



Gill R.'\kers 



The gill arches carry, in addition to the double row of gill filaments on the 



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