216 FISHES OF ILLINOIS 



1.6 in its length; interorbital space 2 to 2 . 5 in head; eye 2.6 to 3.2; 

 nose 2 .8 to 3 .6; maxillary 2.8 to 3 .4; mandible equal to eye; lower jaw 

 slightly longer than upper; teeth in broad villiform bands. 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 Mississippi 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 

 nudes being very small in comparison with the females and fur- 

 nished 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, Julv 15, 1883, contained embryos with prominent eye-spots. 

 Dr. Smith found females with large eggs July 1 in Maryland, ami 

 Aug. 11 obtained others containing young apparently ready for 

 extrusion. Dr. Evermann found specimens containing well-devel- 

 oped embryos at San Antonio, Texas, in November and December, 

 and observations by A. A. Duly, reported by Dr. J. A. Ryder, in- 

 dicate that more than one brood may be produced in a season. 



This little top-minnow, fairly common in extreme southern Illi- 

 nois, has been taken by us outside that region only from Ouincy, 

 Meredosia, and Pekin. Our 1 8 collections are too few to give us data 

 of local distribution, but when treated with reference to the joint 

 occurrence of this species with others more abundant and more 

 widely scattered through the state, they disclose an interesting situa- 

 tion, illustrating the methods by which closely related species occu- 

 pying the same territory come to evade an injurious competition 

 with each other. Bringing into comparison the collection records 

 lor 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) witli nearly three times the relative frequency of the joint 

 occurrence of F. notatusand /•'. dispar, and that i1 occurs jointly with 



