_H. g. gulonella_ is a chubby, deep-bodied fish, whereas _H. g. gracilis_ is long and slender. The head of the creek subspecies is deeper and longer than that of _H. g. gracilis_, being rounded anteriorly when seen in sideview. The head of the large-river subspecies is acutely wedge-shaped in profile. _H. g. gracilis_ has a larger orbit than _H. g. gulonella_. Fins of _H. g. gracilis_ are more strongly falcate than those of the other subspecies. _H. g. gracilis_ has a greater number of lateral line scales, pectoral rays and post-Weberian vertebrae than the creek subspecies. The large-river subspecies attains much larger size than does the creek subspecies (Plate 24). Except in areas of intergradation, complete separation of the two subspecies can be made on the basis of lateral line scales, pectoral rays, post-Weberian vertebrae and head-depth. The regressions of head-depth on standard length in _H. g. gracilis_ from the Saskatchewan River (several localities) and in _H. g. gulonella_ from Beaver Creek, Arkansas River Drainage (KU 4769) are shown in Plate 24. Although values for the largest specimens of _H. g. gracilis_ are omitted from Plate 24, the regression remains essentially linear to standard lengths of approximately 250 mm. On the basis of head-depth alone, separation of the two subspecies is possible in specimens larger than 40 mm. Similar results were obtained by using the regression of postorbital length on standard length, and could have been obtained by using other proportional measurements. NATURAL HISTORY _Habitat_ The species inhabits alkaline streams with shifting sand bottoms where the waterlevel fluctuates considerably with heavy rains and melting snow. The flathead chub is found in silty water and often is the predominant species in streams that have high turbidity. The remarkable ability of this fish to withstand exceedingly high turbidity is illustrated by its predominance in the Little Missouri River, which has an average concentration of suspended silt two and one-half times that of the Missouri River at Kansas City (Personius and Eddy, 1955:42). [Illustration: FIGURE 1. Graphic analysis of lateral line scales, pectoral rays and post-Weberian vertebrae in _Hybopsis gracilis_. In each symbol, horizontal line = range, vertical line = mean, open rectangle = one standard deviation on each side of mean, black rectangle = twice the standard error on each side of mean. _H. g. gracilis_ is found in large rivers throughout its range, occasionally migrating into smaller streams, especially in the spawning season. It prefers the main channel of rivers in moderate to strong current. All series examined are from elevations lower than 3,000 feet. Numbers to left of symbols = number of specimens examined from that locality; combined collections indicated by brackets. The dash-lines represent drainage patterns of rivers in which this species occurs.]