FISHES OF MINNESOTA 3 



the ventrals are thoracic the pectorals are generalh' situated on the 

 sides of the body, as is showu iu the figure of tlie perch, but when 

 the ventrals are abdominal the pectorals are more ventral. The dor- 

 sal, caudal, and anal are sometimes called the vertical or unpaired 

 and the ventral and pectoral the paired fins. 



The length of a fin is the distance along its base and the height 

 is the length of its longest ray. 



The number of rays in any fin is indicated by the arable numer- 

 als alone if soft, but, if it contains spines also, the number is repre- 

 sented by a combination of the Koman and the arable numerals. 

 The number of rays in the dorsal fin of the perch would be indi- 

 cated thus : XIII-I, 14, which signifies that there are thirteen rays 

 in the spinous dorsal, one spine and fourteen soft rays in the soft 

 dorsal. The number of rays is important in the dorsal, anal, and 

 sometimes the ventral, but not in the other fins. 



The body may be covered with scales, as in the perch, or may 

 be naked, as in the bull-head. When the posterior edge of the 

 scales is smooth they are said to be cycloid, when rough or toothed, 

 ctenoid. Most fishes have a modified line of scales, the lateral line, 

 extending along the sides of the body. Each scale in this line pos- 

 sesses a hole or pore which is related to deeper lying sense organs. 

 The lateral line may be entirely absent or only partially developed, 

 and this is sometimes an essential character in the determination 

 of species. When curved downward it is said to be decurved. The 

 number of scales in the lateral line is often very essential. In 

 specific determination it is often essential not only to know this, 

 but to know the number of rows of scales between the front of the 

 dorsal fin and the lateral line, also the number of rows between 

 the lateral line and the vent. A hand lens will be necessary to 

 count the scales in some cases. In the description of the perch the 

 number of scales would be written thus: 5-55-17, which indicates 

 that there are five rows of scales above the lateral line (between 

 the front of the dorsal fin and the lateral line), 55 in it and 17 below 

 (between the lateral line and the vent). These numbers are not ab- 

 solutely constant and those given in the descriptions are generally 

 averages. 



Just in front of the pectoral fins of the perch on each side of the 

 body are the gill openings. The posterior portions of the coverings 

 to these openings are the opercles, which name applies strictly to 

 the bones themselves (Fig. I., Op.). 



Along the lower side of the opercle is the suhoperele (Fig. L, S. 

 Op.). In front of the opercle is the preopercle (Fig. I., P. Op.), which 



