SOUTH AMERICAN NEMATOGNATHI. 43 



DOUBTFUL SPECIES OF TACHISURUS. 



22.1. Arius nigricans Valenciennes, in Voy. d'Orb. ix, 

 atlas ii, pi. iii, fig. 3, 1847; Cuv. & Yal. Hist. Nat. Poiss. 

 xv, 83, 1840 (Buenos Ayres). 



Near Arius albicans C. & V. (= Pimelodus clarias). 

 "Maxillary barbels not extending beyond the shoulders; 

 mental barbels not to the base of pectorals. D. 16 (I. 6?) 

 A. 20. Blackish above, silvery below; fins yellowish." 



22.2. Arius ? obesus Schomburgk, Fishes of Guiana, 

 part i, 174 (Rio Branco). 



"Dull bluish black, paler below. Occipital process 

 large, granulated. Dorsal and pectoral spines serrated 

 on both edges; a spine arises beneath the gill opening 

 and points backward. Anal fin more than usually elon- 

 gate. The teeth are a series thickly set in each jaw. 

 Rio Branco. .18 m." 



ANALYSIS OF THE SOUTH AMERICAN SPECIES OF TACHISURUS. 



a. Head and occipital process entirely covered with skin and muscle; 

 occipital process sparingly granulated. (GALEICHTHYS.) 



b. Anal fin twice as long as high, with 22 rays; fontanel with an indis- 

 tinct groove; occipital process twice as long as broad; gill-rakers 

 3-{-4; light brown, white below; the sides freckled; head 4-4|. 



lentiginosus 23. 



bb. Anal fin higher than long, with 14-16 rays; fontanel continued as a 

 deep groove to the base of the occipital process, which is more than 

 three times as long as broad; gill-rakers 3-J-10: back and sides blue- 

 black, a broad, conspicuous light band along the lateral line; head 3J. 



peruanus 24. 



aa. Occipital process and bones of head usually exposed, skin thin; crown 

 of head and occipital process granular. 



c. Palatine patches of teeth with a backward projecting angle on the 

 inner margin.* 



d. Dorsal plate enlarged.* (SCIADEICHTHYS.) 



e. Teeth on the palate villiform, or bluntly conical. 

 /. Posterior nasal openings not connected by a membrane. 



g. Dorsal plate pointed in front, entering the notched occipital 

 process. 



h. Eye very small, 6 or more in the interocular space. 



* See //. 



