eigenmann: the FRESHWATER FISHES OF BRITISH GUIANA 129 



orbital knobs connected by a low ridge, forming a lyre-shaped figure with the 

 crests extending backward from the orbital knobs, which approach each other be- 

 hind; low crests extending forward from the orbital knob and meeting on the 

 snout, each with a small knob forward of the eye; humeral process extending to 

 the middle of the pectoral spine, a small knob on the sides above and behind its 

 tip; coracoid process extending to the last fifth of the pectoral spine, but slightly 

 converging behind; a transverse ridge joining the bases of the processes, the cora- 

 coid outlined by another ridge. 



Maxillary barbels extending slightly beyond base of pectoral; mental barbels 

 not reaching post-mentals, post-mentals not equal to interorbital. 



Body and head everywhere covered with warts, those on the tail in longitudinal 

 series, those along the lateral line longest. 



Dark chocolate, mottled, some of the warts light; first rays of dorsal, anal, 

 ventrals, and pectoral banded, the rest of these fins black, margined with white. 

 Outer rays of the rounded caudal banded, the rest of the fin black, except the 

 extreme tips of the rays, which are white; barbels banded with black and white. 



Ventrals in the cotype mottled. 



Family III. SILURID^. 



"Skin-fishes." 

 Characters as given in the key, page 119. 



Key to the Genera of Silurid^e. 

 a. Gill-membranes free from the isthmus. Or the nares approximate in Arius. 

 b. Nares approximate, the posterior with a valve (Ariince). 



c. Mental barbels two; maxillary barbel broad, band-like. Pectoral spine, and usually the 



dorsal spine, with long band-like filaments Felichthys. 



cc. Mental barbels four. Palatine teeth fixed. Head and occipital process covered with very 

 thin skin, granular. 



'/. Posterior nostrils connected by a membrane Selenaspis. 



■Id. Posterior nostrils not connected by a membrane. 



e. Palatine patches of teeth with a backward projecting angle on the inner margin. 

 /. Dorsal plate enlarged, the occipital process correspondingly reduced. 



Sciadeichthys. 

 //. Dorsal plate small, crescent-shaped. Occipital process linear or leaf-shaped. 



Notarius. 

 ee. Palatine patches of teeth without a backward projecting angle. Eye above the 

 level of the mouth; gill-rakers twenty-five or fewer. 

 h. Teeth on palate granular; no teeth on vomer; gill-membranes united, a narrow 



membrane free Arius. 



hh. Teeth on palate villiform, vomer with or without teeth Hexanematichthys. 



