"Madsenius" (meaning "Madsen's", for James H. Madsen, Jr., a paleontologist who among other work produced a major monograph on Allosaurus in 1976) is the name given to an as-yet undescribed genus of dinosaur from the Late Jurassic of North America, from the Morrison Formation. It was a theropod which may have been closely related to Allosaurus, or even a species of Allosaurus. The informal name was coined by David Lambert in 1990. Reportedly, it is based on remains once thought to belong to Allosaurus and Creosaurus (usually regarded as a synonym of Allosaurus), and is to be described by well-known paleontologist Bob Bakker as "Madsenius trux".
Lambert, D. (1990) The Dinosaur Data Book, Facts on File, Oxford, England: 320 pp.