1915.] Allen, Review of the South American Sciuride. 171 
In considering the genera and subgenera of South American squirrels it 
seems desirable to take into account also those of North America. The 
number of forms (species and subspecies) of American tree squirrels now 
known is about 175, a number likely to be considerably increased when 
those of South America become as well known as those of North America. 
Nearly as many more occurring in the Eastern Hemisphere are still more or 
less currently included in the genus Sciurus, but many referred a generation 
ago to Scrwrus have since been segregated into a number of groups character- 
ized originally as subgenera. Some of these divisions have come into use 
as full genera, but all of the American species are still commonly referred to 
Sciurus. Scvurus as generally accepted, is thus an unwieldy assemblage 
of several hundred species and subspecies, the considerable structural 
diversity and the relationships of which are concealed under a single generic 
name, although the mass includes many well circumscribed natural groups 
of admittedly superspecific value. The tendency is, on the part of recent 
specialists, to admit more and more of these groups to generic rank, in order 
to express more clearly the interrelationships and the diversities of their 
constituent elements. In line with this trend the present seems a favor- 
able opportunity to call attention to the desirability of eliminating the 
genus Scvurus from the American biota and employing in its place such 
generic divisions as seem properly to express the diversity of the sciurid 
types of North and South America. 
The tree squirrels collectively, morphologically, and in habits, are a 
singularly uniform group, due obviously to their strict adaptation to arbo- 
real life. In Sciuropterus and Pteromys, the so-called flying squirrels, this 
adaptation is modified for pseudo flight, and they are not sciurid in a strict 
sense. The ground squirrels, beginning with Tanmias and Hutamias, and 
including the spermophiles, prairie dogs and marmots, are adapted not only 
to terrestrial life, but have developed burrowing habits, with correlated 
modifications of structure. 
The skull conforms closely throughout the group to a single type, of 
which the European tree squirrel (Sciwrus vulgaris Linné) may be regarded 
as representative. In outline, as seen from above or below, the form varies 
from a broad to a long narrow oval outline; as seen in profile the dorsal 
outline varies from slightly convex or nearly straight to highly arched; the 
orbital fossa varies from subcircular to elongate, the greatest breadth of the 
fossa being usually at or slightly posterior to the postorbital processes; 
the interparietal is widely variable in both size and shape, but is generally 
similar in closely related forms; the maxillary teeth are either 4 or 5 on each 
side, and when p? is present it is greatly reduced in size; the cusps on the 
outer border of the molars; in unworn teeth, vary in prominence in different 
