Family III. Silurid^e. 



i7 



Color yellowish, greenish, or blackish, very variable. Length 12 

 to 18 inches. 



The most southern record for this species is the Rio Grande at 

 Brownsville, Texas. It is the only member of the family found in 

 Mexico which does not have a decidedly forked tail. 



<>. Istlarius Jordan & Snyder. 



Istlarius Jordan & Snyder, Bull. U. S. Fish Comra., 1900, 118. 

 (Type, Istlarius balsamis Jordan & Snyder.) 



Body elongate, somewhat compressed; teeth in villiform bands 

 in both jaws, the one in the upper jaw with an angular posterior ex- 

 tension on each side; band of teeth in lower jaw with a median divi- 

 sion growing narrow and pointed posteriorly ; gill rakers long and slen- 

 der, 17 on first gill arch; barbels 8; skin completely covering the head; 

 supraoccipital bone widely separated from interspinal; humeral 

 process short, almost hidden by the skin; adipose fin with its pos- 

 terior margin free. 



14. Istlarius balsanus Jordan & Snyder. Bagre del Balsas. 



Istlarius balsanus Jordan & Snyder, Bull. U. S. Fish Comm., 

 1900, 118; Rio Ixtla, Puente de Ixtla, Morelos: Jordan & Ever- 

 mann, Bull. 47, U. S. Nat. Mus., 1900, 3138: Meek, Field Col. 

 Mus. Pub. 65, 1902, 72; Puente de Ixtla; Balsas. 

 Basin of the Rio Balsas. (Cuautla; Jojutla.) 



Fig. 2. Jaws showing teeth of Istlarius balsanus Jordan & Snyder. 



Head 4; depth 4%; D. 1, 6; A. 24. Body oblong, deep, some- 

 what compressed; head narrow, not greatly depressed; upper jaw 

 the longer; eye small, 5^ in head; distance from tip of snout to origin 

 of dorsal fin 2^ in body; tips of maxillary barbels reaching slightly 

 past base of pectoral ; pectoral spine slightly serrate on inner margin ; 

 anal fin long, its base equaling length of head; caudal fin forked, the 

 lobes equal; caudal peduncle robust, its least depth 2^ in the head. 



