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 Zoology 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.