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 Zoology 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unpublished data, as for example, proof, in Kellogg's list, that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 Zoology at the University of Kansas
whose untiring aid and guidance has enabled me to terminate this
study.