KEY TO THE FAMILIES OP ILLINOIS FISHES 



Key to the Families of Illinois Fishes 



a. External gill-openings, seven on each side; nostril single, median; no 



paired fins; mouth circular, suctorial; no true jaws 



Petromyzonldae. Page S. 



aa. External gill-openings, one on each side, the gills covered by an oper- 

 culum; nostrils paired; one or two pairs 'of fins not median; mouth 

 more or less obviously a transverse cleft. 



b. Ventral fins, abdominal* or wanting, 



c. Tail evidently heterocercal. f 



d. Body naked or with 5 series of bony shields. 



e. Body naked; mouth horizontal .Polyodontldae. Page IS. 



ee. Body with 5 series of bony bucklers; mouth inferior 



Acipenserldse. Page 2 1 . 



dd. Body with cycloid scales or rhombic (ganoid) plates. 



f. Body with rhombic (ganoid) plates; dorsal fin short (of about 10 rays), 



posterior Lepisosteldae. Page 30. 



ff. Body covered with cycloid scales; dorsal fin long (of about SO rays) 



« AmtidiE. Page 3 7. 



cc. Tail not evidently heterocercal. 



g. A single soft dorsal fin, without spines, except in scaleless forms and in 

 the carp, which has two pairs of maxillary barbels. (In forms with an 

 adipose fin the ventrals are inserted distinctly nearer the anal than 

 the pectorals.) 

 h. Vent behind insertion of ventrals when ventrals are present; body eel- 

 shaped in forms without ventrals. 

 i. Head naked. J 



j. Body more or less completely scaled % (the scales small and sometimes 

 hard to make out in eel-shaped forms); head without barbels or with 

 not more than 2 or 4 (all maxillary). 



*In this key understood to mean that the first ventral ray or spine is inserted 

 evidently nearer to the first (soft) rays of the anal than to the angle under the 

 throat formed by a union of free gill-membranes, or (in case the gill-membranes 

 are not free from the isthmus) to a transverse line connecting the lower corners 

 of the opercular openings. Exceptions to the application of this definition are 

 found in some species of Gasterosteidas, Pceciliidm, and Percopsidm which do not 

 come within our range. 



fThe heterocercal structure of the tail (i. e., the upward bending of the end of 

 the vertebral column) is in all ganoids indicated externally by the obliqueness 

 of the line of insertion of the caudal rays. This line forms a regular crescent, 

 set at right angles with the horizontal axis of the body, in other fishes. In one 

 genus of American ganoids (Amia) the line forms an irregular crescent, which is 

 set, however, at a distinctly oblique angle with the horizontal axis. (Fig. 4-7.) 



t Care must be used here, as the scales are often imbedded, or obscured by 

 mucus. The edges of the scales may be lifted by a needle in these cases. 



§ Except in a few forms, not found in Illinois. 



