My findings agree with those of Nelson in that skulls from Miniatitlan average longer and narrower than those from Altamira, but this seems not to be significant taxonomically because the series from Altamira is, to judge from the material I have seen, somewhat shorter and broader cranially than is "average" for the alleged subspecies _S. a. aureogaster_. For example, series from Metlaltoyuca in Puebla, 3 km. E Axtla in San Luis Potosí, 8 km. NW Potrero and 20 km. NW Piedras Negras in Veracruz, although obtained from localities well within the geographic range of _S. a. aureogaster_ (as outlined by Nelson), all more closely resemble the "topotypes" of _S. a. hypopyrrhus_ in cranial measurements than they do "topotypes" of _S. a. aureogaster_. Conversely, specimens from that part of the range of _S. a. hypopyrrhus_ most remote from the range of _S. a. aureogaster_ (Montecristo, La Venta, and Teapa, all in Tabasco) more closely approximate the Altamiran series in cranial size and proportions than they do the Miniatitlan material. Therefore, my data contradict the statement of Nelson (_loc. cit._) that the skulls of _S. a. hypopyrrhus_ are larger but proportionately narrower than those of _S. a. aureogaster_. The constriction of the auditory bullae alluded to by Nelson as being present in _S. a. hypopyrrhus_ is also present in _S. a. aureogaster_, occurring in both subspecies in varying degrees without correlation with geographic distribution.
Actually, the only concrete evidence of geographic variation that I can detect in these animals is a slight increase southwardly in the frequency and degree of melanism, a kind of variation that is unworthy of taxonomic recognition in this species. It seems best, then, to regard the name _Sciurus aureogaster hypopyrrhus_ Wagler as a synonym of _Sciurus aureogaster aureogaster_ F. Cuvier.
Nelson (_op. cit._:45) stated that _S. a. frumentor_ "Differs strikingly from typical _aureogaster_ in having well-marked nuchal and rump patches of yellowish brown or rufous brown; the underparts gray or gray washed with rufous; tail heavier and more bushy; pelage softer.... Skull indistinguishable from that of typical _aureogaster_." I have examined 22 specimens from Jico, 7 from Las Vigas (the type locality), and one from 3 km. E Las Vigas, all in Veracruz. These include the type and paratypes of _S. a. frumentor_. Part (probably 7 specimens) of the series from Jico was referred by Nelson (_op. cit._:46) to _S. a. frumentor_ and he thought, or knew, these specimens to have been taken _above_ Jico. The remaining specimens labelled as from Jico were referred to _S. a. aureogaster_. I am unable to find fault with the characterization of _S. a. frumentor_ insofar as color or skull are concerned. I cannot verify to my own satisfaction the presence of "heavier" and bushier tail and softer pelage. The characters considered to be diagnostic of _S. a. frumentor_ are distributed in an interesting geographic pattern the genetic import of which is not wholly clear. One specimen (No. 23945 KU) of the two available from 3 km. SW San Marcos, Veracruz, a locality on the coast approximately 50 miles north of Las Vigas, is indistinguishable from topotypes of _S. a. frumentor_ except for slightly lighter-colored grizzled parts. The second specimen (No. 23946 KU) from the same locality, although a subadult in worn pelage, shows the color and striking dorsal pattern of _S. a. frumentor_ and the ventral color of _S. a. aureogaster_. The dorsal pattern of _S. a. frumentor_ is found also in the three specimens from San Carlos and Plan del Río, Veracruz (Nos. 11082, 11083 and 8278), Chicago Nat. Hist. Mus., respectively. (The two specimens from San Carlos were referred to _S. a. frumentor_ by Elliot, Field Columb. Mus., Zool. Ser., vol. 8, Publ. no. 115:128, February 9, 1907.) Nevertheless, although the essential morphological characters of _S. a. frumentor_ occur sporadically in other populations, the animals from the higher elevations above Jico and Las Vigas are notably homogeneous, differ collectively from surrounding populations, and occupy a logical geographic range. Therefore _S. a. frumentor_ is retained as a tenable subspecies, and the animals from the vicinity of San Marcos, and from San Carlos and Plan del Río are referred to _S. a. aureogaster_. Incidentally, Nelson (_op. cit._:45) remarks that he saw no melanistic specimens of _S. a. frumentor_. This is not strange because melanistic specimens could not be identified anyway.