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76 <br />Distribution of castomid fishes <br />Plannelmouth and bluehead suckers were reported in <br />the upper Colorado River System by Jordan (1891). Smith <br />(1959) found both species in the Green River near its con- <br />fluence with the Yampa River. Bolden and Stalnaker (1975) <br />classified the flannelmouth sucker as common in the Yampa <br />River between Craig and Maybell, Colorado, and abundant at <br />all stations downstream. They classified the bluehead <br />sucker as abundant at all stations on the Yampa River. <br />The bluehead sucker, while common at all Pampa and <br />White River samples, probably represents the more "tributary" <br />or "upstream" component of the native catostomid communities <br />of these two rivers. Smith (1966) proposed that the blue- <br />head sucker was the most advanced member of the subgenus <br />Pantosteus, having adapted to the mountain-stream conditions <br />which resulted from the uplift of the Colorado Plateau. <br />This geologic event was apparently complete by Middle <br />Pliocene times (Hunt 1956) and the differentiation of <br />Pantosteus from Catostomus was well under way. Smith (1966) <br />noted the affinity of the bluehead sucker for the areas <br />which corresponded to the former margins of the.Colorado <br />Plateau or to the Plateau itself. <br />Little is known of the zoogeography or geographic <br />evolution of the flannelmouth sucker, but its distribution <br />is usually associated with the swift mainstem segments of <br />streams within the lower Colorado Raver basin. It is