In 1931, C. C. Young named a new species of "Psittacosaurus" for a partial skull discovered in Inner Mongolia, China. The skull was named "P. osborni" after Henry Fairfield Osborn. The validity of this species is now considered equivocal. Sereno (1990) considered it a synonym of "P. mongoliensis", which is found in nearby strata of the same age. You and Dodson (2004) listed it as valid in a table, but not in their text. In a 2010 review, Sereno again regarded "P. osborni" as a synonym of "P. mongoliensis", but noted it was tentative because of the presence of multiple valid psittacosaur species in Inner Mongolia. Young also described the species "P. tingi" in the same 1931 report which contained "P. osborni". It is based on several skull fragments. He later synonymised the two species under the name "P. osborni". You and Dodson (2004) followed this in a table, but Sereno regarded both species as synonyms of "P. mongoliensis"; a table in the latter reported "P. tingi" as a "nomen dubium", however. The front half of a skull from Guyang County in Inner Mongolia was described as "Psittacosaurus guyangensis" in 1983. Disarticulated postcranial remains representing multiple individuals were found at the same locality and were assigned to the species. While it differs from the type specimen of "P. mongoliensis", it falls within the range of individual variation seen in other specimens of that species and is no longer recognised as a valid species. You and Dodson (2004) included "P. guyangensis" in a table of valid taxa, but did not include it as such in their text.