210 FISHES OF ILLINOIS 



Family PCECILIIDiE 



(the killifishes) 



Body oblong or moderately elongate, compressed behind; head 

 broad and depressed; scales cycloid, rather large, adherent; head scaly, 

 at least above; lateral line wanting or represented by a few imperfect 

 pores; skeleton bony; anterior vertebrae simple; fins without spines, or 

 (rarely) a rudimentary spinous dorsal, or a single spine (not in Illinois 

 forms); ventrals abdominal, rarely wanting; dorsal inserted posteriorly, 

 about over anal; caudal not forked; no mesocoracoid; gill-membranes 

 somewhat connected, free from isthmus; branchiostegals 4 to 6; pseudo- 

 branchiae wanting; gill-rakers very short; mouth terminal, small, the 

 lower jaw usually projecting; premaxillary extremely protractile; mar- 

 gin of upper jaw formed by premaxillaries; teeth incisor-like or villiform, 

 sometimes present on vomer, but usually on jaws only; stomach siphonal, 

 without pyloric appendages; air-bladder simple, often wanting; most 

 species oviparous; some forins ovoviviparous, the young well developed 

 at time of birth. 



Fresh-water fishes of small size, widely distributed in Southern 

 Europe, As'a, Africa, and America, Some species occur in bays and 

 arms of the sea, in more or less brackish water. Genera about 35 ; 

 species about 200 ; 2 genera and 4 species found in Illinois. 



Many of the species of this family are surface swimmers, "top- 

 minnows," inhabiting canals, ponds, swamps, and sluggish or stag- 

 nant streams, where they feed on insects and other life found swim- 

 ming or floating at the surface of the water. Other forms (not 

 found in Illinois) are free swimmers in the river channels, and still 

 others dwell in the mud of stream bottoms. Certain species are 

 especially valuable as mosquito destroyers. 



Key to Genera and Species of PCECILIIDiE 

 FOUND IN Illinois 



Fundulus. — Anal fin of the male similar to that of the female, not modified into an 

 intromittent organ; species oviparous. 



a. Dorsal rays 13 or 14; scales 43-45; color olivaceous with numerous dusky 



cross-bars diaphanus. 



aa. Dorsal rays 7 to 9; scales 28 to 36. 



b. Scales 33 to 36. 



c. Sides with numerous narrow lengthwise streaks or rows of dots of dark 



color, the males with dark cross-bars dispar. 



cc. A single black lateral stripe froin head to tail; males with obscure cross-bars. 

 notatus. 



Gambusia. — Anal fin of males modified into a sword-shaped intromittent organ; 

 species viviparous. 



bb. Scales 28 to 30; no evident stripes or cross-bars afiinis. 



