226 Field Museum of Natural History — Zoology, Vol. VII. 



of opercle to base of caudal fin, the transverse series from, the 

 dorsal fin to ventrals or origin of anal, whichever is nearest the middle 

 of the body. In making the transverse count the scale on the lateral 

 line, when it is present, is counted with those on the upper part of the 

 body. The length of the dorsal and anal fins is measured along 

 their bases; the height is the length of their spines or rays. The 

 length of the other fins is measured from attachment to the body to 

 the tips of the longest rays. 



In order to abbreviate, the following expressions are used: "head 

 4" indicates that the head of the fish is contained 4 times in the dis- 

 tance from the tip of the snout to the end of the last caudal vertebra; 

 "depth 4" that the greatest depth (none of the fins being included) 

 is contained 4 times in the same distance; "D. 8" indicates that the 

 fish has a single dorsal fin which is composed of 8 soft rays; "D. iv, 

 9," that the dorsal fin is single and is composed of 4 spines and 9 soft 

 rays; " D. iv-9," that there are two dorsal fins, the first one composed 

 of 4 spines and the other of 9 soft rays. Spines are always indicated 

 in Roman letters, soft rays by figures. The abbreviations used in 

 the count of other fin rays and spines are similarly explained. The 

 diameter of the eye, the length of the snout, and many other short 

 measurements are compared with the length of the head. "Eye 3 

 in head," "Snout 3 in head," indicate that each is contained 3 times 

 in the length of the head. In these particular cases " y^ of the length 

 of the head" would mean the same thing. 



When the last caudal vertebra is about the size of those preceding 

 and it ends in a broad plate (Hypural) which supports the caudal fin, 

 the tail is called homocercal; such belong to most of the true fishes, 

 as the catfishes, suckers, perch, bass, and the like. When the ver- 

 tebrae are progressively smaller, remaining in the axis of the body, 

 and the Hypural is obsolete or nearly so, the tail.is isocercal; such 

 a tail belongs to the codfishes and their relatives. In the heterocercal 

 tail the vertebrae are progressively smaller, and are turned upward 

 into the upper part of the tail. This form of tail is found in the stur- 

 geon, the garpike, and the dogfish. 



In the keys and- descriptions external characters are used so far 

 as possible, our aim being to make identification as easy and accurate 

 and with the least amount of mutilation to the specimen in hand as 

 is practicable. 



To successfully use this paper the student is advised to get a perch, 

 bass, or sunfish and make a careful study of its external anatomy, 

 much as suggested in the accompanying figure, and its description. 



