56 NORTHERN FISHES 



frequently used in identifying various species. The most important ot 

 these counts is the number of scales in the lateral line. Counts of 

 the scales in an oblique line from the lateral line to the middle of the 

 back in front of the dorsal fin and from the lateral line to the middle of 

 the belly anterior to the vent, excluding those of the lateral line, are 

 often used in the identification of various species. In the description of 

 a fish, the perch for example, the number of scales would be written 

 thus: 5-55-17, indicating 5 rows above the lateral line, 55 in it, and 17 

 below it. These numbers are generally not absolutely the same for all 

 members of a species, and therefore averages and ranges are usually 

 given in descriptions. 



The head of the fish is considered to be that portion extending from 

 the tip of the snout to the hind margin of the opercular membrane. The 

 '^percles are the bony flaps covering the gills. In front of the opercle 

 bordering the cheek is the preopercle, the lower bone is the subopercle, 

 and the bone under the preopercle and in front of the subopercle is the 

 inter opercle. For purposes of identification the snout, or nose, is con- 

 sidered to be the region extending back to the front margin of the eye 

 orbit. The cheek is the area lying between the eye and the preopercle. 

 The mouth is said to be terminal when it forms the extreme anterior 

 tip of the head. When the mouth is preceded by a snout, it is said to be 

 suhterminal. The lower jaw is formed largely of the dentary bone, but 

 other bones, such as the splenial, articidare, and angulare, form minor 

 portions. The upper jaw is mainly' formed by two bones, the pre- 

 maxillary and the maxillary. Sometimes a supplementary maxillary 

 may be present. These bones are covered by a thin skin only and can 

 easily be observed externally. The head is said to be depressed when 

 flattened from above, or dorsoventrally, as in the catfishes. 



The membrane on the under side of the opercle is the gill, or branchi- 

 ostegal membrane. This membrane contains a series of supporting bony 

 rays, the branchiostegals. The fleshy space under the throat and be- 

 tween the gills is the isthmus. Sometimes, as in the perch, the gill- 

 membranes may unite and join the isthmus far forward and are said to 

 be free from the isthmus. In other fishes, such as the redhorse, the gill- 

 membranes are joined to the isthmus directly and are said to be united 

 to the isthmus. (See Diagram 2.) 



Teeth may be present on almost any bony structure in the mouth. In 

 a few fishes teeth are vestigial or absent. The teeth of most fresh-water 

 fishes are conical in shape and vary in size from the numerous tiny 

 spines of the bullhead to the large fangs of the muskellunge. Many 

 fishes have teeth, called vomerine teeth, on the vomer bone in the center 

 of the roof of the mouth, and some have teeth on the rudiments of a 

 tongue, although most fishes have little tongue development. Teeth 

 may be located on the p ',htine bones just back of the vomer and on the 

 pterygoid bones located tack of the palatines. Frequently teeth, known 



