128 U.S. NATIONAL MUSEUM BULLETIN 282 



subgenus closest to some of the elongate forms in the subgenus Schilbe- 

 odes, particularly N. gilberti, N. insignis, and A^. nocturnus. 



Ecological considerations. — It is generally recognized that the 

 distribution of flavus is controlled by stream gradient (for example 

 it is absent from southern Illinois, map 8); it prefers riffles or rapids 

 of moderate or large streams which usually have many large loose 

 rocks. It is also known to be common in Lake Erie and Saginaw Bay 

 where there may be a minimum of current, but much wave action. 

 In feeding, it may work into quiet water. Although it prefers large 

 streams I have taken large jlavus in the High Plains from loose sub- 

 merged limestone rocks in the outlet of a spring that was no more 

 than five feet wide and remote from any large stream. Noturus flamia 

 has been taken in collections with gyrinus, nocturnus, exilis, eleutherus, 

 placidus, stigmosus, and miurus. 



The populations of Noturus flavus, particularly, are reduced in size 

 or eliminated in sections of streams where large reservoirs have been 

 constructed. Presumably this is because of loss of riffle habitat plus 

 lowered water temperature. Although flavus is a relatively northern 

 species of Noturus, it is very infrequently found in water cold enough 

 to maintain salmonids. 



Subgenus Rabida Jordan and Evermann 



The subgenus Rabida, a very distinctive group, contains the species 

 of Noturus with serrae (dentations) on both the anterior and the pos- 

 terior edges of the pectoral spine (pi. 4). The anterior serrae are fine, 

 numerous, and usually distinct in all but old specimens. Those of 

 the posterior edge are large and distinctly curved; their tips, except 

 the 1 to 3 basal serrae, are regularly turned toward the base of the 

 spine. The pectoral spine may be short to very long and is usually 

 curved backward, scimitar-like. The grooves of the spine are shallow 

 to moderate in depth and short, mostly occupying the distal half 

 of the spine. 



Other characteristics of Rabida are: body relatively short and 

 usually chunky; lower jaw included; anal fin with 12 to 19 rays; head 

 relatively large; vertebrae 30 to 39, usually 31 to 37; normally 11 

 preoperculomandibular pores in most species, but 10 in eleutherus 

 and hildebrandi lautus; 9 pelvic rays except 8 in the hildebrandi group ; 

 usually 8 soft pectoral rays but 9 in albater and hildebrandi; caudal 

 rays relatively short to moderate in length, the branched rays (12 to 

 24, usually 15 to 19) commonly less numerous than in Schilbeodes or 

 Noturus; usually 12 or 13 vertebrae anterior to the first pterygiophore 

 of the anal fin; precaudal vertebrae variable, mostly 7 to 10; caudal 

 vertebrae extremely variable in number; 5 to 8 ribs on each side; 

 branchiostegal rays typically 9 or 10 on each side; the ossified pectoral 



