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Deinonychus antirrhopus, a new genus and species of theropod dinosaur is described from the Cloverly Formation (Lower Cretaceous) of southern Montana. The species is characterized by a highly specialized pes bearing a very large, trenchant claw on digit II, a highly raptorial manus with a very mobile carpus, and a specialized caudal series with extremely long prezygapophyseal and chevron rods. The new species is closely related to Dromaeosaurus albertensis, which features similar specialization of the second pedal digit, and is referred to the family Dromaeosauridae (=Dromaeosaurinae Matthew and Brown, 1922). Other species with comparable pes structure that may be referred to this family are Stenonychosaurus inequalis, Velociraptor mongoliensis and Saurornithoides mongoliensis.