216 FISHES OF ILLINOIS 



Dorsal rays 6 or 7, the fin inserted behind ventrals; anal rays 8 (females) or 

 6 (males) ; anal fin of males inserted nearer muzzle than base of caudal (vice 

 versa in females), its anterior rays modified into a long, blade-like intromit- 

 tent organ; ventrals reaching to vent; pectorals past front of ventrals, 1.2 

 in head. Scales 28 to 30; .transverse series 8 or 9; top and sides of head 

 covered with large scales. 



G. affinis lives along the southern coasts, in brackish as well 

 as in fresh water, from the Potomac and Delaware to the St. 

 Johns and the Escambia rivers in Florida, and down the Missis- 

 sippi to New Orleans and thence to the rivers of Texas and 

 Mexico. 



Specimens examined by Dr. H. M. Smith were found to have 

 fed on algae, diatoms, and fragments of mosquitoes. 



Sexual dimorphism is strongly manifested in this species, 

 the males being very small in comparison with the females and 

 furnished with a long intromittent organ, the modified first ray 

 of the anal fin. The males are much fewer than the females, 

 68 out of 69 specimens counted by Dr. Smith having been 

 females. The species is viviparous, and a specimen taken by 

 us in Running Lake, Union county, July 15, 1883, contained 

 embryos with prominent eye-spots. Dr. Smith found females 

 with large eggs July 1 in Maryland, and Aug. 11 obtained others 

 containing young apparently ready for extrusion. Dr. Ever- 

 mann found specimens containing well-developed embryos at 

 San Antonio, Texas, in November and December, and observa- 

 tions by A. A. Duly, reported by Dr. J. A. Ryder, indicate that 

 more than one brood may be produced in a season. 



This little top-minnow, fairly common in extreme southern 

 Illinois, has been taken by us outside that region only from 

 Quincy, Meredosia, and Pekin. Our 18 collections are too few 

 to give us data of local distribution, but when treated with refer- 

 ence to the joint occurrence of this species with others more 

 abundant and more widely scattered through the state, they 

 disclose an interesting situation, illustrating the methods by 

 which closely related species occupying the same territory come 

 to evade an injurious competition with each other. Bringing 

 into comparison the collection records for the four species of 

 this family, and taking note of the relative frequency with 

 which the same species have been taken together in the same 

 collection, we find that Gambusia affinis occurs with our most 

 abundant and most widely distributed top-minnow (Fundulus 

 notatus) with nearly three times the relative frequency of the 

 joint occurrence of F. notatus and F. dispar, and that it occurs 



