THE GYMNOTID EELS OF TROPICAL AMERICA. 113 



be elongate Characins without dorsal and ventral fins. However, no intermediate 

 forms are known. 



Gymnotus carapo Linn, is perhaps the most primitive of the Gymnotids as 

 regards the air-bladder, the skull, and teeth. Sternarchus albifrons is, however, 

 more primitive than G. carapo in general shape and in the possession of a caudal 

 fin and a dorsal thong. The ancestor of the Gymnotids may have been a form 

 combining the primitive characters of both these species. 



Key to the Subfamilies and Genera of the Gymnotid.e. 



a. No frontal fontanel; no dorsal filament; no true caudal fin; lower jaw projecting; head depressed; teeth 

 conical, in sockets; posterior air-bladder long, conical; maxillary much reduced. (Gymnotinae). 



b. Anal basis extending around the end of the tail, forming a false caudal; electric organs well developed; 



body not scaled. 1. Electrophorus. 



66. Anal basis not extending around the end of the slender cylindrical tail; electric organs wanting; body 



scaled. 2. Gymnotus. 



aa. Large frontal and parietal fontanels; lower jaw not projecting, or at most very slightly; teeth, if present, 



villiform and without deep sockets, generally placed in patches, maxillary moderate to large. 



c. No caudal fin; tail beyond the anal fin slender, pointed, and usually cj'lindrical; no dorsal filament. 



d. Snout short, not tubular. (Sternopyginae). 



e. Orbital margin free; teeth in both jaws; posterior air-bladder long, conic as in Gymnotimv. 



.3. Sternopygus. 

 ee. Orbital margin not free. 



/. Teeth in both jaws; body much compressed; posterior air-bladder small, subspherical. 



4. Eigenmannia. 

 //. Teeth wanting; body subcylindrical ; air-bladders separate, the posterior cyUndrical. 



g. A cylindrical filament in a groove on each side of the mental region; head chubby. 



5. Steatogenys. 



gg. No filaments as above; head rather pointed 6. Hypopomus. 



dd. Snout produced and tubular; eyes without free orbital margin; very much compressed and 

 elongate; posterior air-bladder small, subspherical. 



/(. Body entirely scaled 7. Rhamphichthys. 



hh. Anterior portion of sides naked S. Gymnorhamphichthys. 



cc. Caudal fin and dorsal filament present; tail rather short; eyes without free orbital margin; air-bladder 



small, subspherical (Sternarchinae). 



I. Snout much produced; teeth in both jaws. 



j. Snout decurved 9. Sternarchorhynchus. 



jj. Snout straight. 



k. Mouth large or moderate; gape reaching at least one-third of the distance to the vertical 



from the eye; snout moderate 10. Sternarchorhamphus. 



kk. Mouth very small; gape not reaching more than one-sixth of the distance to the vertical 



from the eye; snout long 11. Orthosternarchus. 



ii. Snout heavy and blunt, not produced. 



I. Teeth present in both jaws. 



m. Back scaled in front of the origin of the dorsal filament. 



n. Gape long; angle of the mouth little if any in front of the eye; snout prominent. 



12. Sternarchus. 

 nn. Gape short; angle of the mouth not reaching beyond posterior nostrils. 



13. Sternarchella. 



mm. Back naked to beyond the origin of the dorsal filament; scales along the lateral line 



large 14. Porotergus. 



II. Teeth of the lower jaw in a single series; upper jaw without teeth. . . . 1.5. Sternarchogiton. 



