304 Bulletin American Museum of Natural History. [Vol. XXXIV, 
far known. It is an aberrant member of the genus, exceeding the others 
in size, with a harsher pelage and quite different coloration. In cranial 
and other essential characters, as the shape of the skull, the structure of 
the teeth, and the presence of only 6.mamme, it agrees with the other 
members of the genus. It is further peculiar in that it is the only known 
species with 6 mamme that has invaded the great Amazonian area other- 
wise occupied exclusively by squirrels with 8 mamme. It possibly may 
be looked upon as a comparatively recent intrusion from the west of an 
otherwise strictly Andean genus. 
Still another Andean genus of the group of squirrels with 6 mamme is 
the recently discovered Notosciurus (see antea, pp. 209-211), thus far known 
only from a single specimen from northern Ecuador. It is similar in general 
appearance to the hoffmanni forms of Mesosciurus, with which it agrees in 
general coloration and tooth structure, but differs in the form of the skull 
and in the character of the feet. 
The squirrels with 8 mamme comprise two very distinct groups, the 
small guerlinguets and the large so-called ‘giant’ squirrels. The first con- 
sists of the single genus Guerlinguetus, while the other comprises three 
groups of generic or subgeneric value, according to the viewpoint of valua- 
tion. They are here recognized as generic, though they are all much more 
closely related inter se than they are to any other group. 
Guerlinguetus has a wide range, occurring in the lowlands bordering the 
Guiana Highlands, south along the Atlantic coast to at least Pernambuco, 
and in the lowlands of the Orinoco and the Lower Amazon. How far west- 
ward it ranges in the drainage of these two rivers is unknown. It has been 
recorded from the Upper Orinoco as far as the Rio Cunucunumé (near Mt. 
Duida), from the lower Rio Negro, the Rio Branco, the Rio Tocantins, the 
lower Rio Madeira, and the Lower Amazon. In the Brazilian Highlands, 
south to Sao Paulo and Parand, Guerlinguetus is represented by a highly 
aberrant form (Sciurus ingramt Thomas), perhaps subgenerically separable 
from the numerous forms of the Amazonian and Orinoco lowlands. As most . 
of this vast, more or less elevated region is open country, the single species 
of squirrel known to inhabit it is locally distributed, occurring only where 
forested areas offer a congenial habitat. It has a fuller and softer pelage 
than the forms of the other group, somewhat larger size and a somewhat 
differently proportioned skull, differences for the most part plainly correlated 
with the very diverse conditions of environment of the two groups. 
The giant squirrels differ from the guerlinguets not only in size, in colo- 
ration, in the character of the pelage, especially in the possession of a very 
bushy tail, but in the shape of the skull and the character of the dentition. 
The distribution of the group as a whole is also very different, it being almost 
