148 Bulletin American Museum of Natural History. [Vol. XXXIV, 
INTRODUCTION. 
The present ‘Review’ is an attempt to summarize and correlate our 
present knowledge of the Tree Squirrels of South America. The material 
on which it is based, while far exceeding in amount that available to any 
previous investigator of the subject, is painfully insufficient for a satis- 
factory revision of the group, but the author hopes that the results here 
brought together will be an aid to future workers in this field. 
The basis for this undertaking is primarily the large collection of South 
American squirrels in the American Museum of Natural History, the greater 
part of which has been obtained during the last four years by expeditions 
sent out by the Museum to northern South America, under the direction, 
and largely through the efforts, of Dr. Frank M. Chapman, Curator of 
Birds in the American Museum. The purpose of these expeditions, thus 
far carried on mainly and extensively in Colombia, Venezuela, and Ecuador, 
has been the accumulation of material for a detailed study of the bird and 
mammal faunas of South America, with reference not only to the determina- 
tion of the forms occurring there, but as well to their genetic and geographic 
relationships. The present is the third contribution of a somewhat mono- 
graphic character based on the mammals, the first being my revision of the 
murid genus Melanomys, published in 1913,' and the second my review 
of the genus Microsciurus,? published in 1914. It was my intention at 
first to follow the Microsciurus paper with similar reviews of other groups 
of the South American Sciuride, but as the work progressed it was found 
preferable to combine the results in a single paper, and to delay publication 
in order to utilize the collections in prospect from expeditions still in the 
field. In the meantime it seemed best to publish in advance the descrip- 
tions of new forms ® discovered in the material at hand. In order to make 
the present paper a complete summary of the subject, the genus Micro- 
scvurus is here included, but the paper on this genus is given only in outline 
and brought down to date, the detailed descriptions and nomenclatural and 
other comment being omitted; and the same is true of the paper in which 
the various new forms were described. 
. 1 Revision of the Melanomys Group of American Muridee. Bull. Amer. Mus. Nat. 
Hist., Vol. XXXII, pp. 533-555, pl. Ixviii, November 17, 1913. 
2 Review of the Genus Microsciurus. Bull. Amer. Mus. Nat. Hist., Vol. XX XIII, pp. 
145-165, February 26, 1914. 
® Descriptions of New South American Squirrels. Bull. Amer. Mus. Nat. Hist., Vol. 
XXXII, pp. 585-597, October 8, 1914. One genus and 14 species and subspecies here 
described as new. 
