216 U.S. NATIONAL MUSEUM BULLETIN 282 



blotch and the somber color probably were the basis for the initial 

 identification as Schilbeodes eleutherus (see synonymy), but both 

 N. eleutherus and N. placidus differ in the above listed characters and 

 do not have the broad, dark brown or black blotch that is present 

 on the dorsal fin of the hybrid. The pectoral spine (pi. 3, fig. 11) is 

 rather unlike that of any species of Rabida, nor can the greatly de- 

 pressed head be duplicated in any species of the subgenus. 



Noturus exilis seems to be the other parent. It provides the source of 

 the almost terminal lower jaw, the greatly depressed head, and rein- 

 forces the broad dark brown or black margins of the dorsal and caudal 

 fins. A^. nocturnus, in contrast, has an included lower jaw as does 

 miurus and neither of these has the skull depressed. If the hybrid 

 involves A^^. nocturnus, one might expect more caudal rays since both 

 that species and N. miurus have notably high counts, whereas that in 

 N. exilis is low. The serrations of the posterior edge of the pectoral 

 spine in A^. miurus are prominent and uniformly recurved, much as in 

 the hybrid; those of exilis are prominent, but relatively straight (pi. 3, 

 fig. 3) ; those of A^. nocturnus are rarely developed and distinct, they 

 are usually represented by a roughened edge (pi. 3, fig. 5). If the 

 hybrid involved N. miurus and A^. exilis, the prominent serrae would 

 presumably be maintained, as in the hybrid. If N. miurus and A^, 

 nocturnus were jointly involved, the serrae would probably be reduced 

 and irregular, about as in the hybrid A^. gyrinusXN. miurus (pi. 3, 

 fig. 12), an intermediate situation. The nine soft pectoral rays are 

 obtained from one of the dark species, as A^. miurus and most kinds of 

 Rabida typically have eight. The single internasal pore is present in 

 lachneri, exilis, and miurus among the species of Noturus. The pre- 

 operculomandibular pores number ten as in exilis, and are usually 

 eleven in miurus. In nocturnus there are typically eleven preoperculo- 

 mandibular pores in the Arkansas River drainage, but ten pores in the 

 adjacent Red River drainage. Thus, the listed characters point to a 

 hybrid involving Noturus exilis and Noturus miurus. In this case, the 

 ecological distributions of the two parent species slightly overlap or 

 interfinger; one species (N. exilis) is rare at the particular locality, and 

 at the time of the breeding season, isolating mechanisms have 

 broken down. 



Phylogeny 



The North American catfishes, family Ictaluridae, have been re- 

 garded by some workers as representative bagrids and a few writers, 

 even in recent years, have placed them in the Old World family 

 Bagridae. An evolutionary relationship is possible and seems probable 

 but no critical study, involving comparison of fossils, of the divergent 

 Asiatic forms, and of the American species, has been made. Because 

 of several prominent differences, however, it is likely that the current 



