Of _R. parvula_, Nelson and Goldman, on May 19, 1897, on María Madre Island, saved in alcohol an additional specimen (92413 U.S.N.M.) from which J. Biggs, Preparator at the National Museum, in 1951, removed and cleaned the skull. In small size and in all other features, the skull of 92413 closely resembles those of specimens saved by Alcorn from the adjoining mainland of Mexico in Sonora and Nayarit. The pelage of the upper parts of 92413 could be described as "of a light greyish-brown at basal third, fawn-chestnut-brown at apical two-thirds" which are the words that H. Allen (_op. cit._: 285) used to describe the pelage of his _R. parvula_. The external measurements of 92413 are: total length, 60; length of tail, 25; length of hind foot, 5.5; and ear from notch, 11.0. The first two measurements are slightly smaller than the corresponding measurements of any other specimen seen. Nevertheless, the measurements (tail, 30.5; hind foot, 5.3 [after H. Allen, orig. descr.]) of the holotype of _R. parvula_, also from the Tres Marias Islands, show that it was as large as no. 39724 from the adjoining mainland (see table 1). According to the original descriptions, _R. parvula_ and _R. tumida_ differ in size, _R. parvula_ being the smaller. As may be seen from table 1, the alleged type of _R. tumida_ and the alleged paratype of _R. parvula_ indicate the opposite! All specimens obtained since the time of the original descriptions, as may be seen by inspecting table 1, support the correctness of the original descriptions. Therefore, and also because of the other information presented above, I am inclined to the opinion that the holotype of _R. tumida_ and the paratype of _R. parvula_ have been switched; each now is associated with the name and data, at least for locality, of the other. Other opinions are that _Rhogeëssa_ from the Tres Marias Islands average smaller than _Rhogeëssa_ of the adjoining mainland of western Mexico but not enough smaller to warrant subspecific separation of the two. Specimens from places geographically intermediate between the geographic ranges, as now known, of _R. parvula_ and _R. tumida_ probably will reveal intergradation between the two kinds, which, therefore, should stand as subspecies of a single species. As understood now, the Mexican _Rhogeëssa_ are as follows: =Rhogeëssa parvula parvula= H. Allen 1866. _Rhogeëssa parvula_ H. Allen, Proc. Acad. Nat. Sci. Philadelphia, p. 285, type from Tres Marias Islands. _Range._--Tres Marias Islands and western mainland of Mexico from Alamos, Sonora, south to San Blas, Nayarit. =Rhogeëssa parvula tumida= H. Allen. 1866. _Rhogeëssa tumida_ H. Allen, Proc. Acad. Nat. Sci. Philadelphia, p. 286, type from Mirador, Veracruz. _Range._--From Boca del Río, Veracruz, in eastern México, southward over México and Central America into Panamá. =Rhogeëssa gracilis= Miller.