1915.] Allen, Review of the South American Sciuride. 305 
confined to the drainage of the Amazon and its tributaries and the middle 
and upper parts of the Orinoco drainage. So much of this region is still 
unexplored that the continuity of the range is at present largely unknown. 
I have seen no specimens from, nor have I found a record of any taken in, the 
delta region of the Lower Amazon, or even below the mouths’ of the Rio 
Madeira and Rio Negro. Thence northward and westward there is reason 
to suppose that the distribution of the group embraces practically the whole 
of the vast woodlands of the Amazonian drainage, even to the sources 
of its tributaries in the eastern base of the Andes, from southeastern Colom- 
bia to Bolivia, and also across the low divide to the upper sources of the Rio 
Parana. 
Most of the forms of the giant squirrel group belong to the genus Uvro- 
sciurus, with three outlying groups, each consisting of a single species. 
These are Hadrosciurus, comprising the isolated Sciurus flammifer Thomas 
along the Middle Orinoco; Simosciurus (Sciurus stramineus Eydoux and 
Souleyet, and its subspecies), occupying a small area on the western slope 
of the Andes in southern Ecuador and northern Peru; and the Sciurus 
langsdorffit group of the Parand-Tapajos divide and the sources of the Rio 
Marmore in Bolivia. The latter is much less aberrant from typical Uro- 
scvurt than either Hadrosciurus or Simosciurus. It is evident that the latter 
long ago found its way to the western slope from the Amazonian basin to 
the area bordering the Gulf of Guayaquil, and has since become strongly 
specialized through isolation and marked change in environment, shown in 
the form of the skull and the character of the teeth as well as in pelage 
and coloration. 
PHYLOGENETIC CONSIDERATIONS. 
It is perhaps futile to attempt to formulate the phylogeny of the South 
American squirrels inter se. It is, however, pretty clearly evident that the 
Seiuride, including the tree squirrels, reached North America from Asia. 
The marmots have penetrated southward in North America to a much 
less extent than the spermophiles and ground squirrels, which, so far as 
known, have never passed beyond the Mexican plateau. The tree squirrels 
occupy all of North America from the northern limit of tree growth to 
Panama, and extend thence southward through the tree-covered parts of 
South America to 32° south on the Atlantic border, but only to about 15° 
south on the Andean side. Two genera, Microsciurus and Mesosciurus, 
are apparently intrusive into Central America, as there is no North American 
type to which they are closely related, or from which their immediate origin 
can be suggested. As already said, Microsciurus is suc generis; Mesosciurus 
