KEY TO THE FAMILIES OF 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 Petromyzonjdas. Page 5. 



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

 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.t 



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



e. Body naked; mouth horizontal Polyodontidae. Page 15. 



ee. Body with 5 series of bony bucklers; mouth inferior. . Acipenseridae. Page 21. 



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



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



terior Lepisosteidae. Page 30. 



ff. Body covered with cycloid scales; dorsal fin long (of about 50 rays) 



Amiidse. Page 37. 



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). 

 k. Gill-membranes "free"^ from isthmus, i. e., split far forward and meeting 

 in an acute angle. (Fig. 8.) 



I. No adipose fin; belly narrow, carinated; silvery fishes. 



m. Lateral line pi-esent Hiodontidae. Page 42. 



mm. Lateral line wanting'. 



n. Last rays of dorsal fin much elongated; mouth small, low 



Dorosomidae. Page 45. 



nn. Dorsal fin normal, its last rays not elongated; mouth large, terminal, oblique. 

 Clupeidse. Page 47. 



* 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 trans- 

 verse line connecting the lower corners of the opercular openings. Exceptions to the applica- 

 tion of this definition are found in some species of G aster osteidce, Poeciliidce, and Percopsidoe 

 which do not come within our range. 



t The 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 Americ&n ganoids (Amia) the line forms an irreg- 

 ular 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. 

 TT See note under kk. 



