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47 <br />smoothly into a predorsal hump in adults. The hump is not as pronounced as <br />found in G. cypha. The caudal fin is thin and elongated. Fins are large and <br />falcate. The mouth is terminal (Holden et al., 1974). <br />The bonytail chub has suffered the most drastic decline of any of the <br />endemic, large river fishes of the Colorado basin. It was originally distributed <br />throughout the main channels of the lower Colorado River but had become rare <br />by the 1940's (Dill, 1944). Jonez and Sumner (1954) considered them common <br />in Lakes Mead and Mojave when they were first formed on the lower Colorado <br />River, but only occasional specimens are taken now (McAda et al., 1977). <br />The species if now extinct in the Gila River of Arizona (Minckley, 1973). <br />In the upper basin it remained abundant in some areas through the 1960's. <br />Vanicek and Kramer (1969) considered it abundant in the Green River in <br />Dinosaur National Monument, where it has now become rare. McAda and Seethaler <br />(1975) did not collect a single specimen of this species in two summers of <br />sampling (1974-75). Individuals can still be found in the Green River in <br />Desolation Canyon and Canyonlands National Park (Holden and Stalnaker, 1975a), <br />but the size of the population is not known. The specimen discussed (Figure <br />22) above.may be a bonytail chub; however, it is more likely that it is a <br />humpback chub-bonytail chub hybrid. <br />Humpback Sucker (Xyrauchen texanus) <br />The humpback sucker is characterized by an elongate, dorsally depressed <br />head, flattened breast and a prominant dorsal ridge. These characteristics <br />have been hypothesized as adaptations to promote stability and permit upstream <br />orientation in the turbulent waters of the large rivers (Hubbs and Miller, <br />1953). Humpback suckers are large fish reaching maximum weights of 5 kg in <br />the lower Colorado River (Minckley, 1973) and 3 kg in the upper basin (McAda, <br />1977).