419164 Allen, Review of the South American Sciuride. ee 
and occipital region relatively slightly depressed; rostrum and nasals broad, 
the latter well produced posteriorly, forming about 33% of the total length 
of the skull, their length about equal to the interorbital breadth (varying 
from 90 to 110% in specimens of the same species!); zygomatic breadth 
about 58% of length of skull. 
Premolars, ¢; p* with (usually) a strong cusp on fronto-lateral border of 
crown; molars not distinctive. 
Range, eastern United States, south in the mountains of the Mexican 
tableland to Pueblo and southern Jalisco, west to western Arizona, Sonora, 
and Sinaloa. 7 
Species: 6, several with subspecies. _ 
There seems to be no good reason for regarding Areosciurus as separable 
from Parasciurus. The P. niger group is closely related to P. apache and 
P. oculatus (the latter the type of Ar@osciurus) and the other species do not 
differ essentially from niger. i 
The skull of Parasciurus is similar in proportions to the skull in Neo- 
scvurus, Otoscvurus, and Hesperosciurus, but the rostrum is broader and the 
occipital region is much less depressed; the premolar formula is + instead of 
2 and p‘ is heavier and has the anterior cusp much more strongly developed, 
there being four well developed cusps on the outer border instead of only 
three, as in the genera having two premolars. 
The two remaining genera of North American tree squirrels, Meso- 
scturus (Guerlinguetus auct., part) and Microsciurus, are intrusive from 
South America, where they have their principal distribution. The first, 
Mesosciurus, extends across the northern border of South America and 
throughout the Andean regions in the west to southern Ecuador. Its 
range in Central America is discontinuous with its South American range; 
it is known north of the Isthmus from Chiriqui to central Costa Rica, 
with an outlying member (Mesosciurus richmond) in northeastern Nica- 
ragua. The northernmost locality known for Microsciurus is central Costa 
Rica, whence it appears to extend continuously southward in the Western 
and Central Andes to southern Peru and neighboring parts of Bolivia. 
South American genera. 
The South American genera of tree squirrels are in general better circum- 
scribed and more easily characterized than those of North America. They 
may be simply enumerated here, for comparison with the North American 
list, they being described in detail in the following pages. 
Microsciurus Allen, 1895 (subgenus of Sciurus). Type, by original 
designation and monotypy, Sciurus (Microsciwrus) alfari Allen. 
