_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_ × _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.
_Other characters._--Circumference scale-rows 30-40, slightly fewer than in _H. g. gracilis_; predorsal scale-rows 17-27, somewhat fewer than in specimens from Canada, but much the same as specimens from the Missouri-Mississippi system; size small, rarely as much as 130 mm standard length (Fig. 1 of Pl. 24); head-length 24.0-28.0 per cent of standard length, usually more than 25.5 per cent; postorbital length of head 11.2-14.4 per cent of standard length, usually more than 12.5 per cent (both characters illustrate the larger head of _H. g. gulonella_); predorsal length 46.4-52.7 per cent of standard length, longer than in the other subspecies; orbit 5.0-6.6 per cent of standard length; prepelvic length 47.4-53.7 per cent of standard length, longer than in _H. g. gracilis_; caudal peduncle length 17.1-22.7 per cent of standard length, essentially the same in both subspecies.