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A specimen of Sterropterygion brandei, gen. et sp. nov., a rhipidistian from the Upper Devonian of Pennsylvania, shows for the first time the detailed internal structure of the pectoral and pelvic fins and girdles in a member of the Family Osteolepidae. The structure conforms to the general pattern once thought to be directly antecedent to that of tetrapods but which now must also be considered an ancient feature of rhipidistian fishes. It is contended that the known Rhipidistia could not support their own weight during terrestrial locomotion through fin action alone and a scheme of evolution is proposed according to which the paired fins of osteolepids and tristicopterids evolved with a dual function: in locomotion and support of lung ventilation.