Six names apply to the flathead chub, the earliest of which is _Cyprinus gracilis_ Richardson (1836:120). Other names have sometimes been accepted as applicable to valid species and/or subspecies, but usage, diagnoses, and stated ranges have been confusingly inconsistent. For most of the past 100 years, _Platygobio_ Gill has been recognized as the appropriate generic name for the flathead chub, but Bailey (1951:192) places _Platygobio_ and other nominal genera of barbeled minnows having short guts, protractile premaxillae, and four teeth (primary row) in the single genus _Hybopsis_ (Agassiz, 1854). Strangely, the orthotype of _Hybopsis_, _H. gracilis_ Agassiz, is a junior synonym of _H. amblops_ (Rafinesque) (Hubbs and Ortenburger, 1929b:66) and is a younger name than _C. gracilis_ Richardson.
The purpose of this paper is to redescribe the species and to make known its pattern of geographic variation. Natural history will also be considered, as will habitat, food habits, and breeding season.
Ten meristic characters and seventeen measurements of body-parts (the latter expressed as proportions of standard length) have been analyzed. They are: number of rays in the dorsal, anal, caudal, pectoral and pelvic fins; number of scales in the lateral line, before the dorsal fin, around the body and around the caudal peduncle; number of vertebrae; body-depth, depth of caudal peduncle, length of caudal peduncle, predorsal length, length of depressed anal and dorsal fins, length of pectoral and pelvic fins, head-length, head-depth, head-width, snout-length, postorbital length of head, length of orbit, interorbital width, length of upper jaw and width of gape.
Counts and measurements were made as described by Hubbs and Lagler (1958), with the exception of scales before the dorsal fin, which were counted as the number of vertical scale-rows between the upper margin of the opercular cleft and the origin of the dorsal fin. Vertebral counts, made from roentgenograms, excluded vertebrae in the Weberian complex (presumably always four) but included the hypural vertebra.
Counts and measurements were made on series (usually ten fish) from localities throughout the range. To minimize effects of allometric growth, the fish were divided into several length-groups prior to analysis of proportional measurements: 30-50mm, 50-70mm, 70-100mm, 100-150mm, 150-200mm and 200mm standard length and over. The majority of specimens examined were 70-100mm in standard length.