_Pogonichthys communis_ Girard, 1856:188 (in part; original description); Girard, 1858:247 (in part; characters; synonymy); Cope and Yarrow, 1875:653 (characters; Pueblo, Colo.). _Pogonichthys (Platygobio) gulonellus_ Cope, 1864:277 (original description; near Bridger's Pass, Wyo.). _Platygobio gulonellus_ Cope, 1865:85 ("Platte R., near Fort Riley" [Fort Riley is on Kansas R., not Platte R.; Cope's specimens probably are from Platte drainage, on basis of known distributions of other species reported]). _Ceratichthys physignathus_ Cope and Yarrow, 1875:651 (original description; Arkansas R., Pueblo, Colo.). _Platygobio communis_, Gill, 1876:408 (characters; Platte Valley; Green River, Utah [the latter probably erroneous]). _Couesius physignathus_, Jordan and Gilbert, 1882:219 (characters; synonymy; Arkansas R., Pueblo, Colo.); Jordan, 1885:29 (records). _Platygobio gracilis_, Jordan and Gilbert, 1882:219 (in part; characters; synonymy); Cragin, 1885:109 (Garden City, Kans.); Gilbert, 1885:98 (Garden City, Kans.); Jordan, 1885:29 (records); Evermann and Cox, 1896:412 (in part; habitat; synonymy); Jordan and Evermann, 1896:326 (in part; characters; synonymy); Ortenburger and Hubbs, 1927:125 (Canadian R., Norman, Okla.); Hubbs, 1927:75 (parasites; teratology; records from New Mexico); Hubbs and Ortenburger, 1929a:28 (S. Canadian R., Durham, Okla.); Jordan, 1929:76 (in part; characters); Jordan, Evermann and Clark, 1930:136 (in part; synonymy). _Platygobio physignathus_, Jordan and Evermann, 1896:325 (characters; synonymy; records from upper Arkansas R.); Ellis, 1914:62 (characters; synonymy; records from Colorado); Cockerell, 1927:123 (distribution in Colorado); Jordan, Evermann and Clark, 1930:136 (synonymy; records from upper Arkansas R.). _Platygobio gracilis communis_, Simon, 1946:71 (in part; characters; food; habitat; spawning). _Platygobio gracilis gulonellus_, Simon, 1946:72 (characters; records from Wyoming; Arkansas R.). _Platygobio gracilis:_ _communis_ x _gulonellus_, Simon, 1946:92 (North Platte R., Neb.-Wyo. line). _Platygobio gracilis physignathus_, Moore, 1950:87 (habitat; sense organs). _Hybopsis gracilis communis_, Beckman, 1952:50 (characters; food; habitat); Cross, Dalquest and Lewis, 1955:222 (records from Texas). _Hybopsis gracilis physignathus_, Beckman, 1952:50 (characters; habitat). _Hybopsis gracilis_, Eddy, 1957:111 (in part; characters; key); Koster, 1957:61 (characters; habitat; spawning; food); Moore, 1957:110 (in part; key); Smith, 1958:177 (fossil record; Doby Springs, Okla.). _Diagnosis._--Post-Weberian vertebrae 36-38, rarely 39; lateral line scales 42-54, usually less than 50; pectoral rays 14-19, usually fewer than 17; head-depth 13.5-18.0 per cent of standard length, usually 14.8 per cent or more. See Figures 1 and 2.