First of all I acknowledge the encouragement given me in the Proechimys project by Heloisa Alberto Torres, Director of the Museu Nacional, Rio de Janeiro. I extend my thanks also to Stephen D. Durrant, of the University of Utah, for helpful corrections in the preparation of the manuscript; to Mrs. Virginia Cassell Unruh, for the preparation of the drawings of the skulls; to Miss Alice M. Bruce for assistance in drawing the maps; and to my daughter, Julieta, for help in assembling data and for typing.
Dr. Remington Kellogg, Curator of Mammals in the United States National Museum, and the late Dr. Wilfred H. [Osgood], formerly Curator Emeritus of the Department of Zoölogy in the Chicago Natural History Museum, generously permitted me to use their private lists of South American mammals. These lists contain much unpublished data, as for example, proof, in Kellogg's list, that Proechimys guyannensis (E. [Geoffroy] Saint-Hilaire, 1803) antedates P. cayennensis ([Desmarest], 1817). I register here my gratitude to both these zoölogists and acknowledge other critical assistance from Dr. Kellogg.
The John Simon Guggenheim Memorial Foundation awarded me a fellowship for which I am deeply grateful. This expression of the Foundation's interest in education and good neighborliness made possible the completion of the present paper.
Finally I desire to express my deepest gratitude to Professor E. Raymond Hall, Director of the Museum of Natural History and Chairman of the Department of Zoölogy at the University of Kansas whose untiring aid and guidance has enabled me to terminate this study.
[PALEONTOLOGY]
The only known, significant, fossil Proechimys comes from deposits in the limestone caves of Lagoa Santa, Minas Gerais, Brazil. These deposits, of Late Pleistocene or Recent age, were extensively studied by P. W. [Lund] and the results published in a series of French and Danish papers. F. [Ameghino] (1934:110) studied another fauna from a deposit of similar age in the cave of Iporanga, São Paulo, Brazil. Proechimys is recorded in his account under the inclusive specific name fuliginosus.
The molariform teeth of the fossil described by [Lund] (1841:pl. 21, fig. 14) shows its close relationship to the living form P. s. elegans ([Lund]) which still inhabits the same region. It belongs in the more specialized subgenus Trinomys which seems to have been derived from Proechimys. Trinomys has the main fold in the molars always greatly developed and the fold tends to set apart one lamina in the occlusal surface. The Lagoa Santa fossil, like some specimens of the living subspecies, has a small main fold in P4. However, the main fold is large in all upper molars and in the lower molariform teeth which are notably specialized in the extreme reduction of the number of counterfolds to only one.
One hypothesis concerning the evolution of the genus is that a more primitive group of Proechimys lived in all of the Central Plateau of Brazil in the Pleistocene Time. The climatic conditions at that time might have been such as to support large forests but, since the Pleistocene, these climatic conditions may have changed from humid to the present drier conditions, which support the dominant, savanna, floral climax. Actually the extinct fauna from the caves includes animals which have disappeared from the area and now live only in more humid areas, as for example Myocastor, which has shifted to the lowlands to the west and south.
Possibly climatic changes were responsible for the faunal shift from the region that is now a plateau in Central Brazil. This climatic change may have resulted from the gradual uplift of the eastern part of the continent. This uplift prevents part of the trade winds which come from the east from carrying the same amount of moisture inland as they did previously. In fact, the Andean revolution, even if it occurred as late as Late Tertiary, would have had no perceptible influence on the amount of water precipitated on the more eastern parts of the continent. [Oliveira] and [Leonardos] (1943:617) point out that after a Cretaceous submersion of the central part of Brazil, there was a general uplift. The authors (op. cit.:689) mention the presence of continental Cretaceous deposits in the Central Plateau of Brazil, in support of these changes, and state that "pelo menos em certas zonas do litoral a elevação do continente prolongou-se até o Pleistoceno."