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 not 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 profile 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 unessential, it is indicated as " X ". 



The teeth. For explanation of dental formulae used in description of Cyprinidce, 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 theii 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 rays 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 are unbranched, 

 membraneless, closely appressed the one to the other, and in ordinary cases not more than 

 half the length of the fully developed rays. This limitation does not, however, apply to 

 the so-called " club-shaped " short first dorsal ray of certain Cyprinidce (Pimephales and 

 Cliola spp.), which is separated from the ray back of it by a well-developed membrane. 

 The last ray of the dorsal and anal fins is often split nearly or quite to the base and appears 

 as two rays, although counted as only one (Fig. 3). In descriptions, Arabic numerals are 

 used to indicate fin rays and Roman numerals to indicate spines. If a fin contains both 



