160 NATURE-STUDY REVIEW [14:4— April, 1918 



We may begin by directing the student to arrange the fishes in 

 two columns, one containing those in which the ventral fins are 

 abdominal in position and the other those where the ventral fins are 

 thoracic. Thus: 



A. Ventral fins abdominal AA. Ventral fins thoracic 



Horned Dace Stickleback 



Black-nose Dace Yellow Perch 



Sucker Sunfish 



Catfish 

 Brook Trout 



The fishes under A may be further sub-divided by the presence 

 or absence of an adipose fin : 



B. Adipose fin present BB. Adipose absent 



Catfish Horned Dace 



Brook Trout Black-nose Dace 



Sucker 



The catfish may be separated from the brook trout as follows: 



C. Barbels or feelers present, pectoral fin with spine, scales absent. 

 Catfish. 



CC. Barbels absent, pectoral without spine, scales present but minute. 

 Brook Trout. 



In like manner we may divide and sub-divide the successive 

 groups until we have finally separated each species from every other 

 and then arranging the different headings in commonly accepted 

 form, a key similar to the following is likely to result: 

 A. Ventral fins abdominal 

 B. Adipose fin present 



C. Barbels present, pectoral fin with spine, scales absent. Catfish. 

 CC. Barbels absent, pectoral without spine, scales minute. Brook 

 Trout. 

 BB. Adipose absent 



D. Mouth terminal, a black blotch in the lower front angle of the 

 dorsal fin. Horned Dace. 

 DD. Mouth subterminal, dorsal without black blotch, conspicuous 

 lateral stripe, passing through eye and tip of snout. Black- 

 nose Dace. 

 DDD. Mouth inferior. Sucker. 

 AA. Ventral fins thoracic. 



E. Dorsal preceded by four or more free spines, ventrals reduced 

 to a single spine each. Stickleback. 

 EE. Spines in dorsal all joined with a membrane. 



F. Dorsal wholly divided into two fins — Yellow Perch. 

 FF. Dorsal notched but not wholly divided, body deep about 

 equal to one-half the length. Sunfish. 



