The name Rhogeëssa parvula was based on two specimens (Smithsonian Institution Nos. 7841 the type and 7842, in alcohol, collected by Col. Grayson in the Tres Marias Islands off the west coast of Mexico.) The type seems never to have been returned from the Academy of Sciences of Philadelphia to the U. S. National Museum in Washington, D. C., and cannot (in 1951) be found in Philadelphia or anywhere else. The skull, but no other part, of the second specimen is in the United States National Museum under the catalogue number 37329 (old No. 7842). The skull has been broken in two through the interorbital region but is glued together.
Of R. tumida, no additional specimen has been saved, so far as I know, from the type locality, Mirador, Veracruz.
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.