of at least five individuals discovered at the JRDI 5ES Quarry near Grass Range, Montana, and two individuals found in the Meilyn Quarry at Como Bluff. In 2018, new specimen of H. mjosi was described from Montana. Description Hesperosaurus was a large stegosaur, reaching in length and in body mass. Some large individuals may have reached in body mass. In 2001 Carpenter provided a diagnosis. Due to his conclusion that Hesperosaurus were rather basal, in it many comparisons were made with the basalmost known stegosaurian Huayangosaurus, that lost their relevance once it became clear that the phylogenetic position was in fact quite derived. In 2008 Maidment indicated three autapomorphies: the possession of eleven back vertebrae; the fourth sacral not being fused to the sacrum; back plates that are longer (from front to rear) than tall. Maidment also provided some traits in which Hesperosaurus was more basal than Stegosaurus armatus. In the atlas, even in adult specimens the neural arches are not fused to the intercentrum. The postzygapophyses, the rear joint processes, of the rear neck vertebrae do not prominently protrude upwards. In the back vertebrae, the neural arches, above the level of the neural canal, are not especially lengthened to above. At the hip region ossified tendons are present. The ribs are expanded at their lower ends. The neural spines of the tail vertebrae are not bifurcated. The lower end of the pubic bone is expanded (spoon-shaped in side view). To Carpenter this differential diagnosis was problematic because he considered Stegosaurus armatus, the type species of Stegosaurus, a nomen dubium and rejected Maidment's lumping of all North-American Stegosaurus material into a single species, the great variability of which making it difficult to establish any differences with Hesperosaurus. He considered Stegosaurus stenops, the name historically given to several well-preserved specimens, a separate species and provided a new differential diagnosis of Hesperosaurus compared to S. stenops. The antorbital fenestra is large instead of very small. The maxilla is short and deep, half as tall as long, instead of having a height a third of the length. The basisphenoid of the lower braincase is short instead of long. Thirteen neck vertebrae are present instead of ten. Thirteen dorsal (back) vertebrae are present instead of seventeen. The middle dorsals have a basal form in possessing a low neural arch rather than a high one. The cervical ribs have expanded lower ends. In the front