Trachodon — Trachodon (meaning “rough tooth”) is a dubious genus of hadrosaurid dinosaur based on teeth from the Campanian-age Upper Cretaceous Judith River Formation of Montana, U.S.A. It is a historically-important genus with a convoluted taxonomy that has been all but abandoned by modern dinosaur paleontologists.
Interestingly, despite being used for decades as the iconic duckbill dinosaur per antonomasia (see here for a brief discussion on its use in toys and popular culture), the material it is based on is composed of teeth from both duckbills and ceratopsids (their teeth have a distinctive double root), and its describer, Joseph Leidy, came to recognize the difference and suggested limiting the genus to what we would know as the ceratopsid teeth. Restricted to the duckbill teeth, it may have been a lambeosaurine.
In 1856, Joseph Leidy received fragmentary remains from the Judith River Formation, collected by Ferdinand V. Hayden. From these bones, he provided the first names for North American dinosaurs: Deinodon, Palaeoscincus, Trachodon, and Troodon (then spelled Troödon), the latter of which being the only name still in use. Trachodon was based on ANSP 9260, seven unassociated teeth, one of which had double roots. With better remains from Hadrosaurus, he began to reconsider his taxonomy, and suggested, at least informally, that Trachodon should refer to the double-rooted tooth, and the other teeth should be referred to Hadrosaurus. In the Bone Wars that followed, and their wake, the taxonomy of Trachodon and its relatives became increasingly confusing, with one author going so far as to sink all known hadrosaur species into Trachodon except for Claosaurus agilis, but as new material was described from the Rocky Mountain region, Alberta, and Saskatchewan, later authors began progressively restricting the reach of this genus.