EIGENMANN: THE PYGIDIID^E, A FAMILY OF SOUTH AMERICAN CATFISHES. 369 



from snout as from caudal; caudal narrow, obliquely rounded or subtruncate, with 

 few inconspicuous fulcra. 



Translucent; eyes black; chromatophores on the snout, along the back, along 

 the base of the anal, on the base of the caudal, along the side of the abdominal 

 cavity, and a few on the pectoral. 



Genus XVII. Tridens Eigenmann & Eigenmann. (Plate XXXIX.) 



Tridens Eigenmann & Eigenmann, Proc. Cal. Acad. Sci. (2), I, 1889, p. 53. 



Type. — Tridens melanops Eigenmann & Eigenmann. 



Anal long, with twenty or more rays, its origin in front of that of the dorsal; 

 ventrals small, nearer to tip of snout than to base of caudal; head greatly depressed, 

 the eye lateral, infringing on the upper and lower surfaces; a series of fine labial 

 teeth, stronger teeth in the jaws; gill-membranes united, forming a broad, free fold 

 across the isthmus; no nasal or mental barbel, two maxillary barbels; opercle and 

 interopercle armed, the patches of spines separate. 



The two species originally placed in this genus differ so greatly that they should 

 probably be placed in separate genera. The specimens known are all very small, 



27 mm. and less. 



Key to the Species of Tridens. 

 a. Depth 13; head 9; D. 10-12; A. 20-25; opercle with three spines; barbels minute, scarcely evident; 

 distance between origin of dorsal and tip of caudal three in the length; distance between origin of 

 anal and tip of caudal two and five-tenths in the length; caudal rounded, without accessory rays. 



1. melanops Eigenmann & Eigenmann. 



aa. Depth 6; head 6; D. 9; A. 22; opercle with 6 or more spines; maxillary barbel extending to the base of 



the pectoral; distance between origin of dorsal and tip of caudal two in the length; distance between 



origin of anal and tip of caudal less than two in the length; caudal emarginate; eye large, nearer end 



of opercle than tip of snout; first pectoral ray greatly produced. 



2. brevis Eigenmann & Eigenmann. 



1. Tridens melanops Eigenmann & Eigenmann. (Plate XLIII, figs. 1-2.) 



Tridens melanops Eigenmann & Eigenmann, Proc. Cal. Acad. Sci. (2), II, 1S89, 

 p. 53 (lea) ; Occasional Papers Cal. Acad. Sci., I, 1890, p. 339; Proc. U. S. Nat. 

 Mus., XIV, 1891, p. 37; Eigenmann, Reports Princeton Univ. Exped. Pata- 

 gonia, III, 1910, p. 401. 



Habitat. — lea, near boundary between Brazil and Peru. 

 Known from the types, the largest 27 mm., in the Museum of Comparative' 



Zoology, one of which was received by Indiana University in 1891 and bears the 



number 4245. 



Head 9; depth 13; D. 10-12; A. 20-25. 



