l6 NELSON 



contributed largely to the extent and value of recent collections. 

 The bulk of new material, however, has been gathered by 

 Mr. E. A. Goldman and myself. Since 1891 we have been 

 engaged in making explorations for the United States Bio- 

 logical Survey, under the Department of Agriculture. Western 

 Guatemala has been explored and Mexico has been repeatedly 

 traversed from north to south and from sea to sea. The routes 

 of nearly every naturalist whose work is recorded have been fol- 

 lowed, and almost all the known type localities of Mexican mam- 

 mals have been visited. The series of squirrels thus brought 

 together has given an insight into the relationships of the mem- 

 bers of this group, which lack of material had hitherto rendered 

 impossible. Moreover, familiarity with the topography of the 

 country and the geographic distribution of Mexican and Guate- 

 malan species gave me advantages not possessed by any pre- 

 vious worker on Neotropical squirrels.^ 



During the preparation of the present revision I have been 

 able to examine representatives — and in most cases types or topo- 

 types — of nearly all the known species and subspecies found 

 in Mexico and Central America. Dr. C. Hart Merriam, who 

 had already done some work on the group, generously placed 

 in my hands his MS notes and the Biological Survey series 

 of more than 600 specimens ; Mr. F. W. True, Executive 

 Curator of the U. S. National Museum, gave me the use of the 

 National Museum series, and Dr. J. A. Allen, Curator of Mam- 

 mals in the American Museum of Natural History, forwarded 

 the Tropical American squirrels, including the types of his spe- 

 cies, in that collection. These additions brought the total num- 

 ber of specimens up to 919. But after a little study it be- 

 came evident that, in order to reach satisfactory conclusions on 

 many questions of synonymy, definite information was necessary 

 concerning the types of species described by Gray and others. 

 Accordingly, a series of specimens from the U. S. National 

 Museum and Biological Survey collections answering as closely 

 as possible to the imperfect published descriptions, was sent to 

 Mr. Oldfield Thomas, Curator of Mammals in the British Mu- 

 seum, for comparison with the types preserved in London. Mr. 



ij am especially indebted to Dr. T. S. Palmer for n\any valuable suggestions, 

 particularly in connection with the nomenclature and synonymy. 



