210 FISHES OF ILLINOIS 



Family PCECILIID/ZE 



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 rudimen- 

 tary spinous dorsal, or a single spine (not in Illinois forms); ventrals ab- 

 dominal, rarely wanting; dorsal inserted posteriorly, about over anal; caudal 

 not forked; no mesocoracoid; gill-membranes somewhat connected, free from 

 isthmus; branchiostegals 4 to 6; pseudobranchige wanting; gill-rakers very 

 short; mouth terminal, small, the lower jaw usually projecting; premaxillary 

 extremely protractile; margin 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 forms ovo viviparous, the 

 young well developed at time of birth. 



Fresh-water fishes of small size, widely distributed in 

 Southern Europe, Asia, 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 Ilhnois. 



Many of the species of this family are surface swimmers, 

 "top-minnows," inhabiting canals, ponds, swamps, and sluggish 

 or stagnant streams, where they feed on insects and other life 

 found swimming 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 PCECILIIDi^ 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 from 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 lo 30; no evident stripes or cross-bars affinis. 



