class="minor">The following measurements of the skull are used in the tables:

Greatest length: From the anteriormost part of the nasals to the posteriormost part of the supraoccipital.

Condylo-incisive length: From the anterior face of one incisor, at the alveolus, to the posteriormost part of the exoccipital condyle of the same side.

Zygomatic breadth: Maximum distance across zygomata in a plane perpendicular to longitudinal axis of the skull.

Length of nasals: Maximum length of one or both, whichever is the greater.

Interorbital constriction: Least width between the orbits on top of the skull.

Palatilar length: From the posterior face of an incisor, at the alveolus, to the nearest part of the posterior edge of the palatine bone.

Crown length of cheekteeth: From the anterior border of P4 to the posterior border of M3.

In the accounts of species, measurements of aristiforms and setiforms are used. The hairs measured were taken from the middorsal region and outer thighs, and the measurements are means.

All specimens of which measurements are here recorded, as for example in the tables, are fully adult; each specimen shows some wear on each of the four upper molariform teeth unless otherwise indicated.

Capitalized color terms are after Ridgway "Color Standards and Color Nomenclature," Washington, D. C., U. S. A., 1912. One setiform was taken from the animal and placed over the rectangles in Ridgway's charts and the examination made under a microscope with low (×7) magnification and natural light. This method was found to give the most satisfactory results.

The following abbreviations are used for names of institutions:

AMNH—American Museum of Natural History.
CNHM—Chicago Natural History Museum.
DZ—Departamento de Zoologia da Secretaria de Agricultura, São Paulo, Brazil.
MCZ—Museum of Comparative Zoology at Harvard College.
MN—Museu Nacional, Brazil.
MZ—Museum of Zoology, University of Michigan.
SEPFA—**Serviço de Estudos e Pesquisas sobre a Febre Amarela, Brazil.
USNM—United States National Museum.
UZM—Universitets Zoologiske Museum, Copenhagen.

ACKNOWLEDGMENTS

Approximately two thousand skins and skulls were assembled at the Museum of Natural History, University of Kansas, through the coöperation of the authorities in the various institutions of North America, Brazil and Denmark, as listed immediately above. This comprehensive material was used to obtain a more complete understanding of the group, and for the loan of these specimens I am extremely grateful to the authorities of each of the institutions.

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