CXXii FISHES OF ILLINOIS 



Explanation of Terms and Measurements most frequently 

 used in Keys and Descriptions 



1, General Body Proportions 



The length of the fish is measured from the tip of the snout (muzzle) to the base of 

 the caudal rays (end of last vertebra). It does net include the caudal fin, and 

 does not necessarily include the last scales, which in most scaled fishes encroach 

 more or less on the base of the fin . In fishes with a heterocercal tail the length 

 is measured on the median line to the point where that line crosses the line of 

 insertion of the caudal rays. 



The depth of the fish is the vertical distance through the body at its deepest part. 



The width of the fish is taken at the widest part of the body. 



The caudal peduncle, or tail, is the tapering portion of the body behind the base of 

 the last ray of the anal fin. Its length is taken from a vertical from that point 

 to the base of the mid-caudal rays. The depth of the caudal peduncle is taken 

 at its slenderest part. 



The pro-file is the curve from the front of the dorsal fin to the tip of the snout. 



2. The Head and Contiguous Parts 



The length of the head, ordinarily called "head" in descriptions, is measured from the 



tip of the snout to the extreme hinder margin of the bony portions of the 



opercle. It includes the opercular spine in percoid fishes. 

 The width of the head is taken at its widest part. 

 -The interorbital space, or distance, is the horizontal distance on the top of the head 



between the eyes. 

 The diameter of the eye, called "eye" in descriptions, is taken lengthwise, the form of 



the orbit not always being round. 

 The nose, or snout, is measured from the tip of the upper jaw to the anterior margin 



of the orbit. 

 The length of the upper jaw, referred to as "maxillary" in descriptions, is measured 



from the tip of the upper jaw (premaxillary symphysis) to the posterior end of 



the maxillary. 



The gill-rakers are counted both above and below the angle or bend of the gill-arch, 

 the upper number being mentioned first, and rudiments being omitted. The 

 formula 35 + 60, for example, indicates 35 rakers on the upper and 60 on the 

 lower limb ; if the number on the upper limb of the arch is unknown or unessen- 

 tial, it is indicated as " X ". 



The teeth. For explanation of dental formulas used inxlescription of Cyprinida , see 

 foot-note, pp. 102-103. 



3. The Fins 



Fins may be either soft or spinous, or may consist partly of soft rays and partly of 

 spines. The rays of the soft fin or portion are distinguished from spines by 

 their articulated or jointed structure. The peculiar "cross-marks" on the soft 

 ray are, as a rule, easy to make out with the naked eye unless the specimen is 

 very small or the fin jays are covered with thick skin or dark pigment. In 

 cases of doubt the epidermis may be scraped away from a part of the ray or 

 spine and a lens used. In counting the fin rays, rudimentary rays are omitted. 

 Rudimentary rays are those rays, in general, at the beginning of the fin which 



