"Geographic Variation in the North American Cyprinid Fish, Hybopsis gracilis" by Leonard J. Olund and Frank B. Cross is a scientific publication written in the early 1960s. This work specifically addresses the ecological and morphological variation of the flathead chub, a species of fish found across North America, examining its distribution in various river systems and exploring its distinct subspecies. The book thoroughly details the features, habitat, and behavior of the flathead chub, including its dietary habits and spawning patterns. It identifies two recognized subspecies: "H. g. gracilis", which inhabits larger rivers, and "H. g. gulonella", adapted to smaller streams. The authors provide comprehensive data on meristic characters, body measurements, and environmental factors influencing these cyprinid fishes. Through their research, they underscore notable geographic variations related to habitat preferences, physical attributes, and ecological relationships with other species, ultimately contributing valuable insights into the natural history of this fish. (This is an automatically generated summary.)